Imagine a late-game structure along the lines of the Dark Shrine or Templar Archives; we'll call it the Forbidden Records (perhaps balance would dictate having both buildings in place before making the Forbidden Records available to warp.)
This structure would have the research a Protoss player would need to warp in new armaments for gateway units (I picture a warpgate being expended to warp armaments onto an already warped-in gateway unit, for the appropriate cost, similar to evolving zerglings into banelings, or hellions into hellbats.)
This essentially allows player to fill two tasks:
1. Heal units on the field that have taken HP damage, for a cost.
2. Make Protoss less-dependent on "Tier 3" units like the Colossus and High Templar.
This feels like a nice addition to the Protoss "flavor," while also addressing the argument that the warp gate mechanic necessitates the current state of Protoss Tier 1.
In regards to the specific upgrades, they could be something as a simple boost to armor, hp, or shield points, or something more complicated, like a spinning warp-blade attachment that gives zealots a sweeping attack that can hit several units at once, or shields that reduce bonus-damage taken (again, I'm just brainstorming here; the above ideas would all need some SERIOUS balancing.)
*And before you suggest that these ideas are parodying the Zerg's ability to morph on the field, don't forget that High Templar were morphing into Archons FIRST, and the Mothership Core still does it. That's just how the Protoss do.*