I know that the ability to re-do old side/story quests is a feature that is often requested, and I don't disagree that it would be nice (I also don't think that the proposed implementation for 5.0 is what people wanted at all, but that's a different topic altogether). However, a similar request that I rarely see mentioned is the ability to re-do past seasonal events, or complete them for the first time if you missed them. The reason I dislike the current system is that it's a no-win scenario for players.
If past seasonal events were entertaining and had story arcs, then players who have missed them will likely never have a chance to experience them. For example, the past few years' Little Ladies' Day with the Songbirds. The end of the last event makes it clear that that arc is finished, and I think it is a shame that players who missed out on any of the events of that arc will never get a chance to play through it themselves.
It seems like the dev team is aware of this, and increasingly try to make each seasonal event story self-contained. However, this ends up making each event extremely milquetoast and unexciting. Event NPCs give but a passing acknowledgement of your character, and the interactions they had in the past. Character storylines (such as in Valentione's Day, for example), stagnate and refuse to progress for fear of alienating players who haven't done all of the events in sequence. I used to look forward to seasonal events regardless of rewards just so I could see the progression of characters I had fond memories of interacting with, but the past few years I've become apathetic for most of them.
I realize that there are difficulties in implementing a feature like this, especially ones where the cities get seasonal makeovers or there are cooperative activities, but I think it can be managed. I realize that this is a bizarre comparison, but I often think of Granblue Fantasy, a mobile RPG, when I think of the ideal quest system. Not only can every story and side quest be repeated, but old story/seasonal events often get rerun before their follow-up events go live. For older events, they make them into "Side Stories" which vastly simplify the event, strip away any multiplayer components, and make everything into single player instances. I think that FFXIV is more than capable of doing this, given the prevalence of instanced single player content already, and I would love to see such a system put in place. Does anyone agree?