MV was already updated for Trinity, and IMHO it would be absolutely bonkers to update the engine and just like...Roll it back. Especially with such core features almost everyone downloads plugins for the moment they start a game.
So, here's a masterpost of functions that are confirmed, tangentially probably-confirmed, and what from MV Trinity we might be able to expect.
INFO DRIP:
+4 layers of map editing
+Event palette
+

we were starving
CONFIRMED IN TRAILER:
+Offset in character creator
+Dialogue box name options
+Strange tileset hues; probably a tileset hue editor
+Animation speed options
+Visual & Sound effects on battle animation separated
VALIDATED (BUT OTHERWISE UNCONFIRMED) LEAKS:
+ Easy screen and UI size editing, easy font/letter size editing (good for accessibility toggles in options!)
+4 layers of map drawing, can use all the tilesets on every layer. Huge improvement over XP
+Timed battle system with character faces
+Comes parcelled with clickable button plugin
+Comes parcelled with picture-in-text plugin
+EFFEKSEEKER compatible
+PIXI updated
+Battle types are turn, ATB, and Wait Time, and can be used in both side and front view
+Autosave tickbox
WAS IN MV TRINITY:
+ 8-Directional movement
+ Word wrap
+ Editable title screen
+ Battle Results page customization
+ Set Move path visual guide
+ More event triggers (more than double; has stuff like "on player approach" etc
+ FOREGROUND PARALLAX BABYYYYY
INFO WE DON'T HAVE:
-System2 (this covers the vastest range of new functions)
-Animation update
-Exact Range of RTP
-Hinted underlying information not yet indicated by either trailer or leaks
-Where me pre-order