Aside from the UI (and really the whole industry should have taken notes by now), absolutely 0.
They need to keep Persona and SMT completely apart :
-The plot is the obvious one, just look how they went full Scoobydoo and the gang on P4 and imagine that applied to the SMT series. That would be terrible. While P5's was obviously much better (Aside from Akechi's existence), the SMT series needs to stay away from these pop teens and stay true to its origins.
-OST wise, while the incredible vocal performance of P5 should be inspire, again, the whole industry; SMT does not need to go that groovy. Shoji Meguro is amazing on so many different genres, and both SMT main franchise and the various "spin-offs" have different musical identities.
-(battle) Gameplay isn't P5's forte, and I hope nothing from P5's exploration gameplay inspires SMT, SMT again needs to stay true to its roots, more than any other frranchise.
-Characters are of course a no-go.
I don't even know what's left tbh. I don't see anything in P5 that should directly inspire the SMT franchise's future.