Much as I don't want him as Joker, hoping it's not true, I do think you all might be onto something here unfortunately.
The logic of "he's not some one-two-scener" guy checks out. As has been said, he said he's proud of his work on the film, you're probably not saying that if you're like Ramirez in TDK level role. Combined with the fact that while he's hardly a leading-man level actor yet, he's established and on the rise.
The Gotham show theory is the other way it could go that makes sense. He's the lead/co-lead of that, they don't want to say who yet, but he's being introduced here in the film with promises of expansion. I can totally buy what that is.
If he's indeed a rogue being kept secret, revealed at the end, unfortunately I do think Joker's probably the rational conclusion from that. It's who he fits best, much as the choice would bug me. He's got the odd face, the unsettling vibe about him. Ballpark of Pattinson's age (not on board with the idea of young/younger Joker personally, but if you're Reeves & WB it makes all the sense in the world). Probably doesn't command a huge salary just yet so you can get him reasonably cheap before he 50/50 explodes as a popular actor later on.
If he is a villain, you'd figure Joker would be it. Who else would he fit as anyway, Mad Hatter? Not that many options.
*Shrugs* I feel like it's too early for Joker anyway, maybe tease him at the end of a second or even just plain leave him for a third. But pretty sure WB wouldn't see it that way. Could be this is legit, it certainly seemed like one of the more-plausible rumors from way back when during filming.
Super-sneaky-Joker-reveal or lead TV show cop would be where I'd put my money. Hope it's the latter.