I couldn't find a thread dealing directly with this, so i thought i'd make one.
I know many people won't even want to hear about this, but my only aim is to start a healthy discussion about a topic i consider interesting.
1) PK Server(s)
(PK = Player Killing, aka the ability to engage any other player in game in direct combat, eventually killing them)
Even though the initial official stance was that FFXIV was to be mostly a PvE game, how would you feel about having one or more PK servers, added to the current ones, where you can make an extra character (or even your main) and live in a truly dangerous environment, where you can survive only by having either power or friends?
Exp rates could even be lowered so to make it harder to reach high levels, thus creating a broader difference in power between players. Going a bit farther with this idea, periodic character wipes or rare extremely powerful items could further play with the power balance and create even more excitement...
Powerful equipment would also become a very important asset, which would possibly start wars over the resources, making gathering and crafting the center of political struggles. (This factor could be further encouraged by lowering drop rates and creating a shortage of resources.)
Linkshells would become something akin to small nations or brigades, where leadership truly matters for everyone's wellbeing and prestige. Wouldn't that sort of world be instantly both exciting and alive, even with the game as it is right now?
2) Free PvP
Much less dramatically it could be interesting to implement free PvP, where single players or parties are allowed to spar between each other as simple exercise, practice or competition whenever both parties agree to do so.
I think this feature could possibly improve roleplay and solve the hardcore linkshells' bickering among each other, at the same time allowing us to create our own level-capped competitions and tournaments (as we wait for more content, perhaps.)
Finally, implementing this feature would only require coding on the part of the dev. team, without the need to make new SFX, 3d models, textures or complex combinations of content like quests, new cities etc...
There are plenty of models for this sort of feature out there, so it wouldn't even be something so unusual in the gaming world, yet it would immediately attract attention towards the game.