I would personally be fine with Slenderman being in the game, but KingIsaacLinker has a good point. It's not worth the hate that would ensue from rabid internet nonsense-talkers.
To be fair though, those haters would just find something ELSE to hate on, because that's about all they're good at.
Even Slenderman himself, as I heard, is apparently not quite that original either; appears to be greatly inspired by Cabadath, of the Chzo Mythos games (specifically, the third and fourth games). Take one look at that character, and you'll instantly see what I mean.
Having "Slender" in the game here seems fine by me. I think he might fit pretty well, particularly if he can do stuff like vanish and teleport around or something.
Also, that owl there..... he REALLY just doesnt give a crap, does he?