People can make as many crossovers if they like. Even RL crossovers. It's creative, free, and makes everyone happy.
If someone feels weird या any other form of negativity about someone else not them doing them, that's their own problem and the simple solution to that problem is to ignore.
There's a fine line between stories that actually make sense and stories that make me want to sit down and ponder humanity. Half of the fanfictions are the सेकंड kind of story.
So long as the writer can actually find a clever and realistic way to tie the two together then there's no problem in my mind. The problem is, a lot of crossovers don't make a lot of sense.
As a roleplayer and fanfic writer myself, I'm used to crossovers and I use to do them all the time. So I've had my fair share of having to do Alpha and Omega crossovers roleplays with it such as crossing over with them alongside with Planet 51, Brother भालू and even The Emperor's New Groove (don't ask, it was done mostly for fun :P)
