I personally think that there should be, at this point in Batfleck's career, 10 reocurring villains in Gotham: Joker, Harley Quinn, Killer Croc, Penguin, Riddler, Two-Face, Mr. Freeze, Scarecrow, Mad Hatter, and Poison Ivy (if Catwoman exists, she should be an ally, similar to BTAS and Arkham City/Knight). Batfleck could have also dealt with one-timer villains, such as Victor Zsasz and the other members of the Suicide Squad (that is confirmed).
I think they should give one or two of these villains cameos in a solo Batman film, preferably Riddler and Penguin, and mention/Easter Egg the rest (the ones that shouldn't have cameos IMO would be Two-Face and Scarecrow, since they were already done justice in TDK trilogy). The main villain for the Solo batman film to me should be either Jason Todd/The Red Hood (most likely candidate), or Thomas Elliot/Hush, or even the Court of Owls.
Here's my personal cast for the unconfirmed Batman Villains:
Patton Olswatt (is that how you spell his last name?) as the Penguin (he should definitely be done with a similar personality vibe to the Arkham Games)
David Tennant as the Riddler (Or the Scarecrow?)
Bryce Dallas Howard as Poison Ivy (probably like Penguin- a similar vibe to the Arkham Games)
Matthew McConaghey as Two-Face (if they don't easter egg him)
Andy Serkis as the Mad Hatter (he should be easter egged, IMO)
Not sure who should play Mr. Freeze, but to me his suit should be MC, if he isn't Easter Egged.