Seeing other FF series, Yokai Watch, Dragon Quest, Monster Hunter and Nier have their events/contents integrated in FF14, I kinda wish to see an event for Mana series too. People tend to know the series through the SNES game "Secret of Mana", if that rings any bell. The series dated back when Square-Enix was still know as Squaresoft.
The only Mana reference I can dig from FF14 is Watt the dwarf (you know him from the Dwarf Beast Tribe quest). Watt as a dwarf blacksmith in Mana series is a frequent character like Cid the engineer and his airship in FF series.
After decades after no-hit-and-only-miss releases, Mana series finally gains back some traction with Trials of Mana. The remaster of Legend of Mana is not so shabby either, and there are current two Mana games in development (one mobile - ew, and one console). Maybe when the new games is out, SE will consider throw in an event for them?
As a fan of that series, I really hope to see some sort of iconic glam/minion/mount/fight from Mana series as an event.
For minion, Rabite (limbless ball of fluff) is super bouncy and adorable.
For mount, Flammie the feathered dragon (Mana series' traditional flying mount) would be nice. Or a giant Rabite variant.
For glam, I think people would kill for Riesz (an Amazoness from Trials of Mana) green outfit. Her glam also appears in some other games.
For FATE event, the iconic super secret boss Black Rabite would be memorable. Or Anise, who is a dimension-crossing villain of the series, also makes sense the reason she appears in FF14.
I know some people would prefer something original, but I think we already have seasonal events for that, with new stories each year (I kinda wish a way to revisit previous years' stories though).