Persona 6 will probably use the same engine.
Now that they're done developing the engine getting more Persona games out on it will be quicker.
This was my thought too. Index/Atlus realized how much money they could make from Persona games so they wanted to get a really good engine made, Catherine was made to test their HD game capabilities, and now their engine is done so we'll see quicker production of the games. Sorta like how P4 came out pretty quickly after P3 did in comparison to the wait between P4 and P5.
I feel like Atlus realizes Persona is big enough now that it's one of the main moneymakers and thus they will devote the resources to it. That's why thus far P5 hasn't been marketed as "Shin Megami Tensei: Persona 5," like previous entries were, but simply Persona 5.