I've never visited WDW, so I don't know what best would fit there. But I'm a Disneyland local, so here's some of my ideas for my home park:
Bring back Paint the Night. Best nighttime show they ever produced. I doubt it returns to Disneyland, with Magic Happens so prominent and popular, but nothing says it couldn't return to DCA like a few years ago.
Tomorrowland needs the complete overhaul. If that's not in the budget, then at the very least remove the fossil fuels from Autopia, and create sleek, futuristic electric cars. Maybe even some minor interactive elements, like a kiddie obstacle course. Also would love to see Wall-E finally have a presence.
Incorporate elements of the Starcruiser into Galaxy’s Edge. I wanna see aliens and droids wandering around. Have daily Sabacc tournaments (upcharge "entrance fee" is an easy money maker), plus the dinner theater (not sure what to price that, maybe 2 or 3 per night?) Include various challenges throughout the land to assist the Resistance or First Order and access hidden areas (and maybe score some exclusive merch?) Also, finally introduce the promised alternate missions on Millennium Falcon.
Add interactive elements into Avengers Campus. I know there are many possibilities, off the top of my head, we could open "portals" with Dr Strange and America Chavez (with an upcharge sling ring), or build a super suit with Shuri and Riri Williams. Expand Marvel into Hollywood Backlot, with Fantastic four as the anchor. The Baxter building is maybe not stunningly beautiful, but it's still an icon for many comics fans.
Save Avatar for Disneyland forward. I don't want them to just replicate the land from AK, but include locations and characters from all over Pandora. Character meets with the creatures.