Logically, one of the villains in the next solo flick is probably going to be one of Batman's big 5 (Joker, Riddler, Catwoman, Penguin and Two Face). Joker, Catwoman and Two Face have all been used in the past decade and are very recent in memory. It's foolish to say they won't work them in as soon as possible but it is doubtful they'll want Batfleck's first solo flick to draw too much direct comparison to Nolan's Batman trilogy, let alone to The Dark Knight. I think it'll be a long time before we see Ras, Scarecrow, Talia and Bane in live action for similar reasons.
On the other hand, Penguin and Riddler are wide open for another big screen interpretation. By 2019 or 2020, after Justice League when we're most likely to get a Batman solo movie, it will have been about 25 years since we last saw Riddler on the big screen and nearly 30 since we saw Penguin in Batman Returns. There's a lot of opportunity with either of these characters to go in the opposite direction of their 90s counterparts. We've never gotten a really strong cerebral Riddler or mob boss and club owner Penguin.
As such, I'd establish Penguin as the head of the mob and have Riddler come in to stir things up.
Riddler would have the Red Hood gang working for him and one of them would fall into a vat at Ace Chemical thus setting up Joker for an eventual sequel.
I'd include Harleen Quinzel in a cameo as a psychiatrist at Arkham who the police consult with and I'd have her reference Hugo Strange as an offscreen character. I'd also have a scene at Arkham that has a lot of easter eggs hinting at other villains. I suppose my ultimate goal would be to build a 4 movie series that culminates in an Arkham and Blackgate breakout orchestrated by Hugo Strange.