Given there's a number of variations of different Servants in FGO, along with characters being in alternate costumes (see the Tsuki remake, Magical Broomstick Girl Amber, etc), what ideas do you have?
As for myself, I'd love to see Astolfo Alter becoming official in FGO, especially having seen this:
https://twitter.com/xcrid/status/795268162698649601
Astolfo Alter
Caster class Magical Girl Arcueid in FGO anyone?
Doesn't have to be limited to TM though. Perhaps you could mention historical/mythological figures you'd like to see in FGO that haven't debuted yet or sth.