Some things off the top of my head:
-PvP daily quests similar to the beastmen ones
-Larger server groups or just one big group to begin with
-Report function within PvP
-Lower the grind to 30 or make the i90 gear more expensive. You really shouldn't have a full set of i90 waiting for you (and then some) right when you hit r30, though I'd prefer to keep the semi-hardcore grind if possible.
-Maybe a win streak bonus to marks/exp for winning 3/5/7/9 matches in a row
-pvp materia prices should be dropped. They are way to expensive for what they are.

I'll see what else I can think of but that's all I got for now