When Aizen spoke, he spoke of limits in the sense of all a shinigami could learn combat-wise i.e. Kidou, Zanjutsu, and Hakuda(link). He states that no matter how innately powerful a shinigami is, his body and spiritual energy will reach a limit based on the limit that those 3 arts have.
He(Aizen), stated that he had reached said limit because he had developed, to the best of his ability(this is important; not the best possible), these shinigami arts of combat. So for him, hybridization was a way to widen the scope of abilities available to a shinigami/hollow by introducing the abilities of another party.
However, this new entity is not confirmed to have no limits. All Aizen says is that hybridization will destroy the limits shinigami and hollows have; he speaks nothing of the limits the hybrid will possess.
IMO, the hybrid will still carry limits in the sense Aizen spoke of because there will be a point when a hybrid will reach the peak/limit of all the abilities available to him/her.
Actually, "limitless" would mean that they could also redesign the Bleach universe after destroying it lol
Well I've already posted why I believe that eve hybrids are "limited" in the sense of how limits were presented to us.
The potential of everyone is different and is also linked to the motivation of the person. Also being a genius plays a part in potential. Yama is the strongest at this moment in SS (not counting aizen and co.) But it doesnt say he has/had the most potential of all shinigami (yama is just the oldest and strongest at this time) but if hitsugaya is as old as yama he might have masterd the aspects of the shinigami more then yama.
On the hybrid part i am quite sure even the vaizards and arrancar have a limit, and also the arrancar created with the hougyouku arent equaly strong as shown by the espada (this might be because all the hollow used werent at their limits).
Tite kubo was smart enough to use any VL to become arrancar or high lvl captains to become vaizards, as then we could realy speculate on this part.
Whenever bad guys evolve to a higher, more powerful stage, they always gain this one fatal weakness that the good guy takes advantage of. The good guy never ever wins in terms of pure overwhelming kickassery.
Aizen's bankai is doomed to get him killed.
The Living Tribunal. It doesn't make sense to me but to each his own.
Honestly Menos are kind of hax when compared to shinigamis because they derive their power from masses and masses of souls(with one persona being dominant) whereas a shinigami is a singular soul's power. So when we're talking about a VL for example, we're really talking about maybe 500,000,000 souls'(if not more) worth of power clumped into one body.
Nonetheless, I agree with Richo, a given "limit" is different for everyone.
For example, Kenpachi will never really reach the limits of Kidou since he's not arsed to bother learning it. In this sense, his own limit of Kidou will forever be unknown and therefore set to 0 by most readers since his limit isn't pertaining to his ability, but to his own personality.
Another easy example is Orihime. She seriously has the God's power. If she really can reject any event, she could easily reject a person's very existence if she wanted. However, her limit is her personality. Half the time she can't bring herself to use her power so she's in a sense her own limit which maintains the godly power she has within reasonable terms for the manga.
Therefore, when Aizen states he has reached the limit, I think he means he has reached his limit i.e. the most he can develop within the shinigami arts.
Anyway,for some standard we had with "other mangas",and for the "power levels" shown....to be shown.....supposed and potential we have in the bleachverse,I would use the "limitless" to describe a power close to DBZ SSJ 1/2/3....I mean,as strong as they can be Ichigonator/Old Yama (using them cause they basically use raw power),I didn't see something from anyone below and hinted from them,able to let me think:
THEY CAN DESTROY everything around them 100km radious DBZ style....not saying the whole earth or a galaxy.....
So IMO,as far as they don't have ridicolous devastating power,Nuclear size(city destroying and above),saying limitless and very high limit are just the same for the bleachverse....
Isshin - Obvious reason that he's the freak main's father, plus his reiatsu was described as "massive" by Shinji and he only started getting it back!! He could be a Noble too...the resemblance between Ganju and him is too obvious to ignore. And would explain Ichigo's striking resemblance to Kaien. Only thing is he doesn't have the trademark Shiba eyelashes, but seeing as he's a goofball, he could just be plucking them constantly or something. Could also be ex-Royal Guard...he may have defected or something and got replaced by Hikifune.
Ryuuken - Quincy Final Form. Nuff said. Ishida was godly back in the SS arc with that thing even though he was badly injured beforehand and his base form was much weaker than it is now! There's a reason why Quincy's only have one release compared to Shinigami/Vizard and even some Arrancar. And a reason it comes at such a high cost. Complete dominance over spiritrons is the second strongest power in BLEACH, after Orihime's
reality warpingphenomenon rejection. Yes, even stronger than Kyouka Suigetsu IMO. Ryuuken's base form is already WAY stronger than Uryuu's so when he uses that release, his opponents (I don't care how strong they are) are gonna get their shit wrecked.
Yoruichi - Man, where do I even begin? She has godly written all over her! Mentor to Soi Fon and Byakuya - two powerful and capable warriors in their own right. Leader of the strongest Noble clan (http://www.mangatoshokan.com/read/bleach/1/-106/6), and the first female leader too. That has to mean something special. Has the title "God of Flash" with apparently signature Shunpo techniques at her disposal. We've only seen one so far (Cicada) but they are supposed to be 3 IIRC. Going off her title, she should be the fastest character in the manga when she gets her skills back in top gear. She's been around pretty much since the start of the series and we haven't even seen her Shi Kai yet. Heck we've barely seen her zan at all and I seriously doubt thats because she doesn't have use for it. Soi Fon uses hers all the time so I expect the same from her master.
...I didn't include Urahara in my post cause honestly I don't expect him to be as flat-out strong as these 3. His reiatsu will certainly be high (we already know it is actually), but I think his main strength is his mind. His genius and innovation in battle will allow him to match or even surpass people that are technically "stronger" than him.