I think one of the most disappointing aspects of the primal fights (beyond the style of item acquisition) was the method of entering the instanced arena. It seems the majority of the game's content (primal fights and raids) are all accessed via a dull, featureless cave. What sort of build up is that? I'm assuming it's mostly due to the focus on 2.0's world design, but I would like to suggest something on that note:
Primal Temples
Basically, open world mini-dungeons of sorts which would house the waypoint to the primal fights. Imagine large cathedrals, designed with their corresponding beastman's architectural style in mind, full of fire/wind/moogle based themes and symbols, chock full of high priests, guards, and perhaps an NM or two guarding their precious waypoint within.
I find this an excellent way to combine interesting open world content with instanced boss fights, something for everyone to enjoy! Besides, those featureless caves need to go and I hope that either way, something a bit more exciting is done with them in 2.0