These are just my ideas, just to keep them in mind and to dazzle you. Also, by the way, maybe they give you some good ideas or suggest that I add a character here or release another one.
2nd Holy Grail War
Saber: Yamato Takeru
Lancer: Nagao Kagetora
Archer: Aśvatthāman
Rider: Kyokutei Bakin
Caster: Ashiya Doman
Assassin: Dante/The Grim Reaper
Berserker: Sun Wukong
(Now that the last chapter is out along with the cover, you should already know who the rest of the servants are, so I'll just list them).
5th Holy Grail War
Saber: Artoria Pendragon
Lancer: ???
Archer: Emiya
Rider: ???
Caster: ???
Assassin: ???
Berserker: Heracles
(I'm only going to make one Servant change here.)
The London and ??? Holy Grail War
Saber: Baldwin IV
Saber: ???
Lancer: Gareth
Lancer: ???
Archer: Arjuna
Archer: ???
Rider: Perseo
Rider: ???
Caster: Tamamo-no-Mae
Caster: ???
Assassin: Carmilla
Assassin: ???
Berserker: Asterio
Berserker: ???
???
???
(I already have them all ready here; I'm just not showing them so it's a surprise, but I can still make changes.)