I have been underestimating Doflamingo's importance in the story for a while now, but thinking about it, his character has such a significant role within the current world of One Piece:
In the event of Doflamingo being defeated by Luffy (plus allies) and the SMILE factory being destroyed, the world really will suffer a tumultuous shift:
- The most powerful broker in the underworld, "Joker", and the middleman for most of the deals that go on within the underworld;
- The middleman between Kaido and Caesar for SMILE;
- The King of Dressrosa;
- Current owner of the Mera Mera no Mi which is being used as the reward for the Corrida Colosseum;
- Former Tenryuubito;
- Member of the Donquixote Family;
- Captain of the Donquixote Pirates;
- Ally of Caesar Clown
- A Haoshoku Haki user;
- Doflamingo being the "most dangerous man" within the Shichibukai means that the influence, intimidation and terror he brings to the Shichibukai powers are unmatched by any of the other members. Law even stated that if Doflamingo resigned, the Marines would be forced to send their top fighters, the Admirals, after him. Now if Doflamingo's Shichibukai title is revoked once he is defeated by Luffy, the immense damage it would to do to the worth and significance of the Shichibukai as a pirate-deterring power would be immense. I highly doubt that the "pirate" who replaces Doflamingo's Shichibukai seat would bring as much to the table as Doflamingo did. As illustrated in chapter 700, the Marines are already starting to panic at the prospect of losing both Doflamingo and Law from the Shichibukai (Law's title has already been revoked). Even Sakazuki seems somewhat uneasy about the events going on.
Furthermore seeing how Doflamingo already resigned as a Shichibukai that day to have it later announced as being a mistake/a false report by the Cipher Pol 0, once Doflamingo is defeated, a second update will need to be made expressing how Doflamingo has really been removed from the Shichibukai. CP-0's credibility will be negatively hurt by this, especially if Doflamingo has to give up his Shichibukai title.
With Issho's expressing his intention of wanting to abolish the Shichibukai, after the truth about Doflamingo concerning the suffering of Dressrosa citizen's surfaces, the Shichibukai may very well be abolished after the current year's Reverie if the Kingdoms/nations request the abolition of the Shichibukai as well - how Crocodile, Blackbeard, and Doflamingo severely disrupted nations and how Doflamingo has been supplying other countries with weapons encouraging countries to wage war. I guess in the event of this, the World Government won't need to worry about replacing Doflamingo =P.
- Also being the most powerful broker in the underworld, "Joker", Doflamingo has tremendous influence in that world. We have found out that he has been related to supplying countries engaged in wars that have been increasingly arising within the New World, with weapons. Doflamingo's defeat and potential crumbling of his role as "Joker" within the underworld when he loses power within Dressrosa will be catastrophic to the parties involved in deals with him. Some wars may have been engaged on the foundation that they had the support of "Joker's" underworld trades. Once "Joker" is gone and those countries are unable to power their military might with the weapons they get from "Joker", how exactly will things play out after? Which countries will be left once the dust settles? And beyond countries, how many pirate crews dealing with "Joker" will be affected negatively?
- Providing SMILE for Kaido which is presumably one of the reasons why Kaido is such a notable force within the New World (one of the Yonko). Doflamingo has shown to be terrified of Kaido and is prepared to do whatever it takes to prevent the supply of SMILE from being stopped, even going so far as to sacrifice his own crew members and to use his connection as a former Tenryuubito to fool 10 people. One can imagine how furious Kaido would be if Doflamingo fails to deliver on his side of the deal once the SMILE factory is destroyed. How much will this weaken Kaido's forces and influence within the New World? This could lead to an increase of pirate crews/factions/powers challenging Kaido for the position as one of the Yonko.
Once Doflamingo is defeated, likely a big fissure within the underworld will open up after "Joker" disappears. I am curious to see if Law will try and fill that hole and take on an active role within the underworld to help The Strawhat Pirate Alliance and their plan of taking down Kaido (and all the other Yonko) - the Yonko operate in the shadows/underworld, so getting information through normal means would be ineffective, I guess this is also why Kuzan is connected to the underworld, in order to get information. Once "Joker" is gone, someone or some group of individuals will inevitable rise to take on the position "Joker" used to occupy in the underworld, so it may as well be Law as a way for him to be a major player within the Strawhat Alliance.
- As the King of Dressrosa he has significant influence and intimidation - one of the reasons why he is such a valuable member of the Shichibukai. Once he is defeated and control over Dressrosa goes back to the Riku Family, the whole of Dressrosa, including the Tontatta Tribe, will have their freedom back. With the Riku Family back in power, it is likely that they would help lead surrounding countries affected by outbreaks of war back to the states they were in before Doflamingo took sovereignty over Dressrosa. If pirate mercenaries like Hajrudin are a big thing in the New World, having countries move away from warfare could affect those mercenaries as well. This may also have impacts in the upcoming Reverie. And who will Dressrosa be affiliated with after this arc?
- The implications of the Mera Mera no Mi power coming back into play will surely cause waves throughout the New World for whoever wins/steals/acquires/eats it. Depending on who eats/acquires the Mera Mera no Mi, it could also be symbolic of Ace's will being carried on and thus the waves washing across the New World could be massive - I'm sure the World Government wouldn't want something like that happening at this chaotic point in time.
- Luffy defeating another Tenryuubito, well Doflamingo is a former one, may infuriate the Tenryuubito even more as to how much disrespect this "low-class" existence has towards them. They are already pissed Luffy punched Saint Charloss and hasn't been apprehended, so seeing him do the same to a former Tenryuubito may force them to take matters into their hands instead of solely relying on the Marines. Luffy a potential #1 enemy on the Tenryuubito list? So far Fisher Tiger and him seem to be the only ones who have taken such direct actions in opposing the Tenryuubito (not acknowledging their superiority), well there was also Zoro on Sabaody Archipelago, but Jewelry Bonney stopped him...
- I'd imagine the Donquixote Family may have something to say about Doflamingo's current situation. The Donquixote Family may be concerned about the damage Doflamingo will do to the family name once he is defeated. They could make a move on Luffy at some point after he defeats Doflamingo (causing the "Donquixote" name to be damaged/tarnished).
- Once Doflamingo is defeated, what exactly will happen to him? Will he be arrested like Crocodile or will he disappear like Moriah? The former would mean that the Donquixote Pirates are without their Captain and potentially would disband if not gather together to try and rescue their Captain. Would Doflamingo follow his intention of becoming the Pirate King once all other distractions, such as being the underworld broker "Joker" and middleman to Kaido and Caesar, are gone. After a climatic encounter with Luffy, Doflamingo may let go over his fear for Kaido and seek to exist within the New World without going under the protection of a Yonko or anyone else. Luffy may very well remind Doflamingo about the romance of adventure he felt from Roger during Roger's execution 24 years ago.
- With Doflamingo defeated, Caesar's role in producing SAD for him will end, but will other powers in the New World seeking the power of SMILE let Caesar's scientific ability pass them by without doing anything? Even if Doflamingo is defeated, there will still be two Yonko after him - Kaido and Big Mom. Would the Strawhats and Law really hand Caesar over to the Marines after they destroy the SMILE factory in Dressrosa? Do they trust the Marines enough to keep Caesar in check without another major incident occurring due to his scientific ability? Caesar Clown has one frightening scientific mind, so it may not be in the best interest of the Strawhat Alliance to let Caesar go. I can see Law taking on the duty to watch Caesar and keep him in check.
- What will be the significance of Doflamingo wielding the Haoshoku Haki? Those who wield such power are said to be contenders for becoming the next Pirate King, would Doflamingo embrace this possibility and finally journey after One Piece once he is defeated by Luffy? As a pirate free from the World Government, Shichibukai and the Yonko, he may finally recall why he desired to be the Pirate King once (if he had let go of that dream at some point that is).
What is certain though, the world is going to change significantly when Doflamingo is defeated. Law was correct when saying "D" will bring forth a storm and when he told Doflamingo this:
Yeah. Dd is a bigshot with BIG connections. Luffy is gonna kick his butt and dd will feel it from many different places (kaido, dealers, etc.)
About the first point I think that nobody will replace Donflamingo because the shichibukai system is going down.
About the 6th point, Donflamingo may be the son of a member of the Gorosei (still not 100% confirmed), Luffy is infuriating a lot of big names here.
9-Big Mom is already looking for Ceasar and Donflamingo has yet to be defeated , I think Ceasar will be with the heart pirates/strawhats for a while (don't know about his destiny)
10- Sealing for One Piece would be nice but will he have such freedom? He is really powerful but at least one Yonko will be looking for him (Kaidou).