Outside of the story and main dungeons they bring, I'd suggest;
- Swimming & surface movement on water
- Underwater capability for our Magitek mounts...
- extend flying to some of the ARR zones
- Permit swimming around the beaches in Costa Del Sol and The Mist
- No new species, but deeper customization options for the ones we have
- A tank that uses magic, not physical weapons
- A dual wielding max DPS stance for PLD (and a similar all-out DPS stance for other tanks) that sacrifices some defense for maximum damage output
- A sword based DPS (Samurai would be cool)
- A blunt weapon DPS that uses hammers and maces
- A physical healer - heals with medicines and such, not just a spell.
- New Jobs should start at 30, with slightly fewer skills than MCH, AST and DRK had, to allow an easier introduction, but add the reamining skills more quickly as you level so by 60, you are on par with others.
- Up the level Cap to 70
- Slow the item level growth so it does not spin out of control
- More horizontal gearing
- Fix secondary attributes so that we can create alternate builds that vary performance of our job in meaningful ways. Something that is more meaningful than everyone stack Crit, and then Det...
- Add at least one more cross skill slot and open an additional job for choosing cross skills
- Allow one of our retainers to accompany us in the field as a field companion to fight along side us, though when they accompany us, they can undertake no ventures or manage the market board sales.
- Give us more inventory space or a 3rd free retainer (especially if you allow a retainer to become a field companion).
- Add some mid-level dungeons with puzzles and other fun, rather than punishing, mechanics.
- Enforce stricter level sync to mitigate the ridiculously overpowered characters in older content. Arum Vale is a strict-ish level sync which maintains some of the difficulty and requires people to pay attention to mechanics, this should be how level sync works on all level 50+ content as well, but with a slightly looser ilvl sync than AV uses.
- Remove all of the running around from point to point filler quests from the ARR and HW MSQ, and re-distribute the XP, gil and gear to other MSQ quests. Maintain all quests that actually contribute to the main story, but remove all the filler to allow a swifter progress through the MSQ.
- The long slog of getting through ARR's MSQ all the way to Steps of Faith in order to get to HW has acted as a deterrent for RMT in HW zones, this is one major reason why there should be no skipping of the story. Collapsing all the filler quests that involve no encounters shouldn't make it any easier for RMT to get to the new content since all the dungeons and other trials still have to be completed.
- Please give us the sleeping emote our characters use at the Inn when we log off there. It's 100,000,000,000 times better than the 'doze' emote is for Miqo'te...
- Expand on the ideas in the Deep dungeon and the treasure map dungeon, there are innumerable doors and gates that go nowhere in the game. drop in a few more of them and then use them as entrances to procedurally generated dungeons for dungeon crawling. The entrances should be found using a different kind of map from the treasure map. Once discovered, the entrance should always be visible to the map holder, until they have completed the final encounter successfully and obtained the final treasure. Such maps should not always lead to a functional entrance, and sometimes the dungeon beyond should be very shallow and in ruins preventing further exploration, in these instances, the map would turn to dust in the player's hands.