I'd like to see Proposition Treasures that you could actually equip and Unexplored Lands you could actually visit and find bonus dungeons.

Also I'd like to expand monster breeding so you could cross-breed different monsters to produce even more powerful hybrids that could be used as allies or poached for even rarer stuff.

Zodiac Stones that could be used in battle to spectacular effects. Maybe morph allies into powerful monsters that differ on who equips it.

Terrain that shifts during the battle. Imagine a fight during an volcanic eruption or on ice floes. It would add a whole new level of challenge to battles.