I know this is a ton of money and a big dream but Disney needs to think BIG!!!
I'd add/change a few bits if we're thinking big:
- Reopen BB for one more season. Whilst its open, plan it's replacement. By the time it closes for next season all the equipment/rides have been fabricated and a team is ready to go with demolishing it, to replace it with a new water park approx 3x the size to rival Volcano Bay / Aquatica, which have vastly more exciting attractions. Whilst thats going on TL will have extended opening to counter BB being closed (much like they are having to do at the moment).
- AK has the most land available for expansion, and is the park with the most guest space. Rip down dinoland and start again, expanding it with a larger land (dont really care what the theme is, they can figure that bit out). It'd need a good 2 or 3 E-ticket attractions. I'd also do as you say and add another attraction over at Pandora to help level the crowds. Really they could do with something new over in the Asia section as well.
- Epcot would get a couple of new pavilions in the world showcase, this doesn't need to be too fancy, just something to absorb extra guests for a while. Play Pavilion would also be finished off. The Seas also needs an overhaul, it's feeling very tired.
- I like the idea of New Orleans for MK, although not sure how BTMRR would fit in there as you'd be exiting Frontierland, going into New Orleans and then have an obvious Frontierland attraction in the middle of it. Perhaps just sticking to converting Tom Sawyer Island into something would be more feasible.
- HWS could probably be left alone, it's got enough demand with SWGE and TSLand. At most the retheme of RnR could be done on the cheap - no alterations to the ride at all, just rethemed to something a little more up to date.
I honestly dont think a 5th park is on the cards ever though. They're massively struggling to keep enough staff in place to run the ones they've already got.