This seems like something that makes the most sense out of any possible collaboration.
Both games represent the nostalgia of multiple games from the series while still strongly representing their own stories. Both games are huge money makers for the series so the fan base has to be there. FFBE already had raids with FFXIV primals and the Scions as units to use. It would seem like genius cross promotion for both games and a no brainer to actually bring it over to FFXIV.
Contextually they can do whatever for the actual quest, but if they want to actually bring over specifics they could just have some NPC talk about finding some strange crystal somewhere and seeing someone appear wearing an "odd looking uniform" that doesn't match any of the city states that we have to investigate. They could then say they think they might be Garlean to provide a risk and reasoning for us to check it out.
After we find the crystal, we then come across the person who it turns out to be Rain and he is looking for his friends (Lasswell and Fina). We then go around other places fighting some battles or whatever to rescue Fina or possibly fight Lasswell as a misunderstanding situation. Then we all join up with a happy reunion and it is then revealed that they are just "Visions" from the crystal.
For event items, glamour sets of Rain, Lasswell, Fina + chibi minions of them. I would say weapons but they are too specific and people would feel left out if they only did the 3 of them.
There are a lot of other FFBE story characters glamour that could be brought over if they even made it a yearly event. And it would even open the possibility of including the weapons since most of the jobs could be covered by that point.
- Lid, Nichol, Jake, Sakura
- Dark Fina, Raegan, Sol, Akstar, Charlotte if needed
- The Veritas glamour would be epic.
- Sworn Six season 2.
Unlike the other main line games, FFBE is ongoing as well, why not fully support the obvious advantage of cross promotion for something that actually makes sense and is still hugely successful in it's own right and helping the company hand in hand.
I honestly thought the Lightning event and even the Shantotto event were more of the type of things we could look forward to but that all seemed to die out pretty quickly. Even the Iroha event was back in 2015. I don't understand why they don't embrace the actual series more.