Two of the currently most popular MMOs already use solutions mentioned in this thread:
Final Fantasy XIV: one character can do everything
World of Warcraft: a character can freely switch between multiple different talent specs
Furthermore, there are interesting lessons in the development history of both games.
FFXIV has struggled with the idea of specialist crafters. At first, there was a very steep cost to switching specializations, and the system received a lot of negative feedback. Development moved away from specialist recipes and the mechanic seemed all but scrapped. More recently, they’ve brought specialist recipes back, but they’ve dramatically decreased the cost of changing specializations. This system seems to be a much better compromise between players who want specialization and players who want to do everything.
WoW originally did not have multiple talent specs, and when they were first introduced it cost gold to unlock your second spec in-game. Last I checked, now multiple specs are unlocked by default (no extra cost) and it’s easier than ever to switch on the fly.
To throw in a third example, the original Guild Wars also made it easier and less costly to switch skill sets through its development.
I think these examples show a clear trend towards players preferring to do everything on one character - if not at once, then at least with a free and easy option to switch back and forth. You see the exact same trend in this game with how players use alts. The only major difference between an alt and a swappable skill set is you have to change names and do some minor extra micromanagement. In that case, swappable skill sets would be a great quality of life change without impacting the current economy or how the game is played.
I’d personally vote for two or three sets that you can switch between anytime you’re in the sanctum, with no cost, no cooldown, and no penalty.
I understand that some players worry it would negatively impact the economy or social aspect of the game to allow a character to do too much at once. In my long history of playing MMOs, I’ve never seen anything to suggest this would be the case. In fact, I’ve consistently been able to make money in several different games just by selling vendor items for more than I paid.
That tells me that there are enough different playstyles and priorities among players to create meaningful trade even if every character can do everything. Not everybody wants to do everything. People will happily pay to avoid the parts of the game they don’t find fun, and luckily, there are always players who do find that part fun and are happy to profit from it.