I do believe the time has come for the next KoA to join the party. We saw how potent Percy is, and we are made sure Sin and Donny are almost like trash at this point. So another arc of Percy humiliates the enemy doesn't seem to add anything new to the story.
Although I can't still rule out the possibility of Nasien being a KoA, I believe Nakaba picked the name because he has connections to Percival in the legend. Nasien also does not look like a typical Nakaba protagonist. By the looks of it, he is a side character at most.
(I still can't figure out how Nakaba came up with the name Donny tbh.)
But for who may join next, I think things would become too complicated if they reach Liones this soon. Liones is the first big country Percy is going to visit, and the universe suddenly expands to include not only Tristan and even the SDSs, but several other elements completely new to the story.
I don't say it's impossible, but an intermediate arc between the initial ones and Liones arc will make things move forward more smoothly.
For that I thing the next character should better be a KoA to speeds things up a bit, but not Tristan himself. My guess is in fact War. War is the most worn out and the least majestic of all the KoA portrayed in the manga so far. (Yeah the depictions are not accurate, but still better than my own pure fantasies.)
Finally, I believe the order by which they join Percy is already decided:
Percy; Death; verdant wing-like hair! (The order in visual depiction and verbal description match perfectly.)
Tristan; Pestilence; holiness and evil in the eyes (again the placement of the character is the panel and the order by which he is described have a especial relation.)
If the pattern actually means something, then we would have this two other sets:
???; War; mysterious lad with no fixed appearance (most probably Lance because of similarities with Arthurian legend.)
???; Famine; Golden magic (maybe Gawain >50%, maybe not<50%.)
So the hypothetical order would be like this:
1. Percy,
2. War,
3. Tristan,
4. Famine.
There are too many IFs here, but the huge amount of uncertainty is what makes speculating funny for now. Soon we will face with Nakaba's real plans, and there would be no room for any speculations.