As of now most of the decently rewarding content is based on 8 people parties.
Not saying it's bad, but it often results in lots of random people recruiting or long long standing by doing nothing but waiting for 7 linkshell members to log on.
Personally i find myself waiting for my LS mates to gather for longer than I actually play with them...
This issue could be easily solved by adding content for 3-4 people party.
Having something to do with couple of your best friends in game while waiting for the rest to log on would make this game much more enjoyable.
Not to mention that bonding experience with few people in party is a lot stronger.
Not asking for anything fancy. Actually can list a few simple solutions:

1. Changing rewards - Simply change rewards (drops) for killing monsters that are not soloable.
This will encourage people to invite a couple friends to farm those drops. To make it long lasting experience i would recommend putting some nice consumables as a reward, so players would want to come back there again. (Potions, remedies, elixirs, food or even maybe anima potions)
To avoid RMTs overcamping the place rewards should be untradable.

2. Quests - involving instanced NMs (or spawned only by party that has the quest) to avoid competition, hard enough to be challenging for 3-4 players, rewarding with special consumables at the end. Could involve multiple NMs in one quest as long as the reward is worth it.

3. Quest lines - series of quests available one after another, involving various NMs, higher level monster farming for untradable drops. Quests shouldn't be soloable, but should limit party members to ~4. Since quest line would be a lot more time consuming it should have matching rewards! For example unique untradable materia that can't be obtained in any other way or even unique untradable weapons that can be spiritbonded to unique untradable materias. Letting a player to choose unique (materia) reward from the list at the end of the quest line is a good idea too imo.

Note how all the rewards should be unique and untradable. This will not only help avoiding RMTs abusing this content, but also encourage all the players to do it, instead of buying the rewads from other players, making players a lot more active at times when their LS doesn't have enough online players for Full Party content~

I really hope to see some of the above mentioned small party content on 2.0+ :3
Feel free to share your ideas on this topic and correct me if i overlooked something