So far I'm having a lot of fun with the new Trust NPC system, and I think it has a lot of potential. I do have a few suggestions I'd like to make though.
1) Allow Trust NPC's to be summoned in all zones, including Shadowrealm zones. Perhaps add quests that will unlock access to Campaign, Besieged etc.
2) Allow Trust NPC's to be used in player parties - not just solo play. This way, a party could include any combination of Trust NPC's and players up to a maximum of 6 party spots (but do not allow alliances). This would be beneficial for players who need help with game content but are unable to find enough interested players.
3) Add resting and non-combat stances to Trust NPC's, so they draw weapons when you engage an enemy, put them away when you disengage, and rest when you rest, just like Adventuring Fellows do.
4) Add some combat speech to Trust NPC's. It'd be fun to see them talk while fighting. It'd be cool if NPC's could respond to eachother's speech, but that'd probably be too much trouble.
4) Add another limit break quest for Adventuring Fellow NPC's, raising the cap to level 99.
5) Add a final quest for level 99 Adventuring Fellows, which upon completion, will eliminate the need for signal/tactics pearls, and will transfer your Fellow to the Trust NPC system. This way, the Adventuring Fellow will be summonable by magic, and will appear in party with you instead of a separate HP bar. You would still be able to interact with your Fellow at Rendezvous points though, and choose jobs etc.
6) Allow for more job options for Adventuring Fellows. Perhaps add quests to unlock each regular player job for Fellows to use, and maybe even add quests for getting job-specific AF gear for your Fellow.
7) Add special solo BCNM battlefields for Fellows and Trust NPC's. For example, there might be a BCNM where you can enter with your Fellow, and choose a maximum of 2 Trust NPC's to assist. Rewards from these battlefield events could include gear and weapons for your fellow, perhaps rare Trust ciphers, among any number of other things.