To repeat my earlier points: As long as we're talking about a (mentally) stable and healthy person, I'd say very little. It might enhance some traits they already have (personal example: I've always had a "leader-like" personality and once I actually started to fullfill the role as tank within the game this might have been slightly enhanced), but I would expect it to be the other way around: Based on your personality you pick a job that fits it. Your person-character molds your ingame-character, not the other way around.
The whole point of roleplaying games - wether its you actually immersing yourself and pretending to be someone else or if its only playing as a different character through their story - is kinda to be someone else for a short while. Not to become someone else. Maybe you'll find crumbs of wisdom within games and the odd thing you can apply to your personal life (just how you could find those crumbs in books, shows, films, cheesey quotes on facebook...) but overall there shouldnt be any effect from games to your personal life - MMOs are no different in that regard.
You dont become a protector of the weak in real life just because you picked up the paladin-crystal last night. You dont become an angerfilled beast because your character nos posses the warrior-soulstone. Its not even like that within the game itself, after all...
Gaining social connections and being able to interact and communicate with a lot of different people all over the world might be very beneficial, specially to those who struggle to make bonds in real life. So thats the only real influence I could see an MMO specifically have on your life and potentially your personality.
Soulstones? Soulstones are just a game-mechanic with some lore attached. Not some life-changing artefacts.
If they are that for you, maybe you're in need of a change in your life indeed... but not in one that involves soulstones, but rather in one that involves less games...