Gear sets. In 1.x, certain pieces (AF) had synergistic traits to the job they were related to. Though Yoshi had said they were taken out to simplify things, I think they should make a return. However, let's bring them back in a similar way the GC gear implements them but instead of adding pure stats in the way those pieces do, have them boost certain abilities or traits. For example: Allagan and Mythology sets could have different types of bonuses allowing more reasons to mix and match the sets based not only on stats but the bonuses you would receive for equipping them.

Though certain pieces may not be considered BiS, so long as you are hit capped, i90 is i90. There is no content I can't clear because I have 1 i90 item and not another. Point being, sub stats don't make a big enough impact to drastically negatively effect any of the content.

Example: How cool would it be if the 5 piece Valor set bonus gave 2 secs more to Hallowed Ground (doesn't have to be this but using it as an example). And say the 5 piece Allagan tank set reduced the CDs of Hallowed Ground/Holmgang by a certain amt of time or even something broader to encompass both jobs. There would also be 2 and 3 set piece bonuses as well.

Yes, it would take balancing but I think it would help breath a bit more life into the gear variety. All jobs would have set bonuses relative to their respective roles.