Without any limitations? Massive amount of dev resources into player centric modes and their updates. PvP and Mahjong are already pretty solid but Lords of Verminion and Chocobo racing are REALLY outdated. Maybe even new game modes that use collectables like TT cards, Orchestrions, Minions and Mounts to encourage people to collect, even sightseeing log or fishing stuff could be used as base for something. If done properly, these would be extremely replayable content that could help with the end of patch moments where people don't have all that much to do.

If I had to choose addition that is more reasonable, then I would add more PvP and Mahjong rewards. Mahjong could easily use Top 100, give plenty of MGP for general playing (since the games tend to go kinda long) and maybe have slightly more adjustment to Rank based on the final score instead of pure finishing postion (tentative, more experienced people should be referred wheter this one is good or not). PvP itself is on right track, CC is quite good despite the netcode being what it is, Rival Wings deserves a roulette and Frontline rework is on the horizon with 7.0.

I would add things like PvP Relics that cover all the game modes in some ways, new set of hellhound weapons and hellhound armor set to spend collars on (and commedation crystals to upgrade, imagine the hellhound set getting more magnificent the more pieces you augment with these, having the final full set being a great longterm goal and something to flaunt) and open up more things to be bought with Wolf Marks and Trophy Crystals. Just adding Hunt stuff to their Vendors or even a new ranked currency that covers all the ranks but scales based on the final rank as alternative way to obtain some rewards from other content would be great.

If I had to go minimalistic in the additions, I would just add recolors from PvP Series rewards into Reaching X Ranks (ie. first recolor at Platinum or Diamond and second at Crystal or Top 100), allowing everyone to get the base version but encouraging people to play Ranked for alternate colors (this includes Mounts and the PvP set mainly, maybe minions?) also increasing the overall rewards for winning or losing enough to make Ranked appealing option to grind instead of Casual.