My pick would be to change things to have Sega either:
a) have Yakuza 6 be the last game on the old game engine
or
b) have a spinoff Yakuza game release as the first Dragon Engine game, and 6 be released a year later...
Because while I like 6, its definitely harmed in several ways by being the test bed for the brand new game engine, with several features (like a big part of Kamurocho) being completely absent from the game. It releasing in the state Kiwami 2 did, or just with 5's combat/engine would have been better no doubt.
Oh, and while I'm at it, I'll go back slightly further and stop them from nerfing the hard modes on the games after 3.