[MILD SPOILERS for those not caught up on Gotham ahead!]
Gotham showrunner Bruno Heller made it clear before the show began that it would be a while before viewers at home would know who becomes The Joker in his TV version of the Batman universe mythology. However, when you draw out a mystery as tantalizing as that, you invite fans to come up with their own elaborate theories about the answer... and a more popular one (oddly enough) is that Oswald Cobblepot a.k.a. Penguin (Robin Lord Taylor) is really the future Clown Prince of Crime.
You can read the finer details of this Gotham fan theory over at Reddit. Part of the reason this idea has resonated with so many fans, of course, is because the TV series has already played rather fast and loose with the traditional Batman mythos (to great, terrible, and/or mixed results, depending on who you ask), having characters meet and/or interact in ways unprecedented in the history of stories examining what life is like in the eponymous city (across multiple forms of media).
It's for that reason that Taylor, when asked about the Penguin/Joker theory by MTV, made it known that he very much doubts it's a valid one... but also admitted that he can't rule out the possibility of such a revelation happening on Gotham one day.
“I find it incredibly unlikely, but at the same time, I have no idea what could happen. I get the scripts and I’ll read something, and I will be just as shocked as everyone else is when they see it happen onscreen. [I read that theory], and I was like, ‘Huh.' I was like, ‘Nah.’ But then I was so impressed at the detail that this person went into… the guy who formulated this theory. I was just amazed at the amount of attention that people are paying to the show. I mean, he was looking at the colors of my vest and my suit, and extrapolating off of that.”
This particular Gotham fan-created theory probably don't even seem worth even considering, for some (many?) of those who've been watching the comic book-based crime procedural.
... Then again, before the series began, you might've never believed someone who theorized that we would see young Bruce Wayne (David Mazouz) go on adventures in Gotham City with young Selina Kyle (Camren Bicondova) and/or cross paths with the future Poison Ivy as a kid (Clare Foley). Heller and his writing staff, as mentioned before, are certainly taking some liberties with their portrayal of Gotham citizens in the days before the rise of Batman - could one famous Batman villain wind up being an amalgamation of two well-known ones? (... Yeah, probably not.)
One of the primary story threads (and many would agree, most successful) on Gotham thus far has been about the young Mr. Cobblepot and how he's played the mob bosses of Gotham against one another, while slowly accumulating more power and influence for himself. If thing keep evolving the way they are, Gotham will have (if nothing else) given us perhaps the most interesting, crafty, and compelling version of the Penguin in any storytelling medium to date.
The Joker generally possesses all of the aforementioned qualities (in adding to those violent tendencies of Taylor's Oswald), yet the patience and power-playing of Oswald on Gotham doesn't necessarily gel with what Joker usually represents - disorder and anarchy, among other things. Then again who knows, maybe Heller and his fellow writers/producers really are trying to pull one heck of a fast one on fans (by way of some rather subtle foreshadowing) right now.
... Though, that might be giving too much credit to a show where Edward Nygma (Cory Michael Smith) seemingly can't go a single episode without asking a riddle or using a coffee cup that doesn't have a large question mark on it...
Gotham continues with "What the Little Bird Told Him" on January 19th, 2015.