Something... new.
Wouldn't this be the first option? A Planar Source Book to give all the various options?I'd like an 'All of the Above' book.
A chapter on each of the planes, with the mechanics for the planes spelled out.
A chapter on Sigil.
A chapter on Spelljamming.
A chapter on Planeswalkers and how they interact with the Great Wheel.
A chapter with new character options for PCs intended to walk the planes.
A chapter with magic items and spell from the planar cosmology.
An appendix with monsters.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.