Perhaps a system like a blizzard has. Account name, which would be the name we are active under in forums by default (and could be changed once, if people wished, because they didn't plan on it being forum name, so give everyone one inital possibility to change it if they like, then keep it), and then character names in game would be independent of it.
But friends could be found through account name.
For example: my account name would be Gothix but I wouldn't need to have character named Gothix in game, I could have demon named Demonicus, and beastman named Beasty, and people could still find me by searching my account name (Gothix).
Of course if people wished to played incognito, privacy option would exist to be turned on, so people who don't wish their characters known can't be found through account name.
Initially, system could set account name to already existing forum user name, and allow people one free change, if they wished, as long as what they wish isn't taken already.
This could work, I know Path of Exiles has that sort of thing i think, I added a friends account name and now when he is online it shows up the persons account name and current chars name hes playing so i can fire him over a private message in game and send him party invites etc via the social tab, and yes if you wanted to be incognito then that should be made available.
I have always used the same char name for my main and i have even bumped into people after years of not having contact with them as they reconised my name of choice going back 16+ years. I usually always play the same sort of play style. Healer of some sort on most games that have healer so the naming is a big thing for me.
I know a lot of people the name isnt really a big thing but there are people who i think the naming is very important for.