An IP that you pretty much know won't get a fighter because the source material just doesn't allow itself a good transition?
Two come to my mind, and it's funny, because both IPs are fighting games, which Smash doesn't seem to have issues with, but trust me, these two IPs are such cases.
The first one is Darkstalkers. There're cases where this is debatable, but generally Smash leans into protagonists.
This is a problem in this case. You thought Mai Shiranue from Fatal Fury had a complicated design to censor, Morrigan straight up doesn't even try, she's off-limits.
And Demitri has a different problem, being probably his most iconic move Midnight Bliss, where they turn the opponent into a female, chokes them, drains their blood before incinerating them (burning the clothes as well, with some nudity in some cases as a result of that) and just when they hit the ground they go back to normal like nothing happened, which got issues of age rating (due to being somewhat explicit), company guidelines (because some won't tolerate the transformation of their characters) and also straight up being hard to do (Demitri rarely makes it to crossovers cause of that move alone).
The second example might be even more controversial though, as I don't remember anyone asking for Darkstalkers rep in Smash...Mortal Kombat however...
Yeah, the main thing going against this are the fatalities.
Such a trademark of the franchise, but what makes them work is A)the one doing the fatality and B)the one on the receiving end getting eviscerated AT BEST. If what I said about Demitri before serves as any indication, this is Demitri's problem but super amplified because pretty much everyone in Mortal Kombat has a brutal fatality.
Anyways, any other example comes to mind? Comment.