There are already many FATEs that chain together leading up to one big FATE that drops rewards and achievements (Formidable and Archaeotania in ShB for example). I do hope they add more like those two in the next expac, but maybe make them a bit easier to track - it's a bit sad when you follow one of the chains for 20 minutes only for it to reset back to the first one because the final boss one was already spawned a few hours ago.
I don't think they should be soloable especially if they drop exclusive items, there are more than enough ways such as linkshells, PF or Discord to gather enough people to take one on and it gives more people a chance at getting the rewards. Even ones like 'Special Tarasque Force' in dravania gets people to come and help, even though it just drops an achievement and a triple triad card you can get from beating an NPC.
Maybe a bit of voice acting in FATEs could be good, but I'd rather they just add more voice acted stuff to MSQ and side quests, plus maybe not many people would hear it over the sound of hundreds of spells and attacks going off lol