Since its release in 2013, Payday 2 has grown into a vast, lore filled universe, one that has ties to John Wick himself. However, there is only so much you can do to keep a game up-to-date with expansions and dlcs. At one point you have to introduce a sequel or continuation of a live game.
The same seems to be happening with Payday 3, as the next game will now be moving to Unreal Engine for development. UE is easily one of the most influential and versatile game engines available today, levelling the playing field for both Triple-A and indie studios.
Overkill revealed this info via a cryptic tweet, that came with a first glimpse at the game, potentially showing off Bain. Not much else is known, but it will be interesting to see where Overkill takes the franchise in the future.
However, we might have to wait for while as it does appear that it is still in the early design phase, meaning that the whole concept of the game is being written and prototyped. It is only after this that extensive in-engine development will take place.