yeah, he plays as a regista, but these two guys have similar characteristics. for me, locatelli is a more reliable rabiot, so the regista position suits him a bit better than rabiot, but i'd try him as a mezz'ala.
i also get what you're saying: he also looks like a natural arthur replacement in our current system. in that case, we could move arthur to the mezz'ala position, the one he played at barcelona, but his complete lack of forward passing doesn't really suit him that well.
for me, arthur and locatelli could play in the middle. either way, we would need an attacking minded mid too. for me, locatelli and rabiot would be redundant.