Some content for small party (2-4 people) with decent rewards from it to keep the interest. Apart from exp grind there's nothing to do if you want to have fun with a friend or 2 without having to form Full Party and do raids/primals. In my opinion small party content is important to make close bonds with people.
Those bonds might save this game if it continues to provide small content updates as it was till now. Having close friends in game and constantly having something to duo or trio with your best friends can keep a player online way longer even if there's not enough content - friendship can keep a player interested. As it is now, however, forming Full Party often results in having random people in your party that you don't know at all and don't have as much fun with, while its forces you to get to know other people it wont be as bonding as duo content would be for example.
Simple solutions starting from least complicated one:
1. Increase rewards from monsters/NMs that are not soloable or are very hard to solo. Don't even have to make new content - just use the higher level monsters, give him some nice drop rate of useful material/gear (that can be traded for seals) etc. so a person would have a hard time soloing and he will be forced to look for a friendHave to be wise on deciding on the reward though to keep this interesting and fun and last longer (might want to keep untradable rewards to avoid RMT camping)
2. Creating NM's that are hard to solo, but cannot be spawned if you have too many people in party (3-4 people limit)
3. Using above mentioned NM spawn system in a small, repeatable quest line that after beating all the NM's would reward with usable, untradable items, something that everybody needs and would gladly invest time for it, some unique potions that are not obtainable other way that give big boost in battle or even Anima Potion i bet that would get looooots of people interested!Something that would be worth coming back and repeat the quest line again with your best friend when there's not enough people for full party.
4. Instances even if small and take only 15 minutes to complete with your friend. Doesn't have to be nothing fancyGive few nice rewards, maybe consumables that are worth coming back for and let the friendship continue~
Long story short - lots of people come to have fun with friends in game, not just grind solo or recruit for hours a full party - have to give them something to do if there are only few good friends online!
Thumbs up if you think these thoughts should be shared in separate thread and ill look into making one, or you can do that if you please ;D