Very engaging conversation.
I have a different prospective on Xp, that doesn't seem to have been discussed yet.
XP, is three things primarily; its a currency, its a reward, and its time.
My experience, is when players are demanding/asking for Xp roll over for a new character, is that they don't want their time on the game to be invalidated. They've invested a lot of time on the game, and thats shown in a few ways. One way, which is (in most cases) forever lost, are the IC relationships, and the narratives therein. The second way, is the amount of XP they acclimated. Since XP is also a reward, they have an entitlement to that reward. Simply making a new character shouldn't be invalidating their work for their reward. The time they've placed into the game.
Players didn't mind the early game for the mu*. Thats why they're old character with lots of XP. If the early game was crappy, they wouldn't be old characters. (Though maybe the game has changed where the early game does now suck, but I'll assume it doesn't.) So Players more then likely wouldn't mind doing so again, rebuilding those IC relationships, and driving those narratives. As that basic game loop is very fun.
So its not having to go through the 'tutorial' again. One, they wouldn't be. It'd be a new game+ experience, and get to skip any tutorial like element. Two, they would have passive advantage that wouldn't be represented in the game, verses new players. Their game knowledge. Which is something often overlooked and under appreciated.
Now, as most of this thread has concluded, you can't really allow players to keep the XP. It creates a lot of problems.
The biggest problem, is XP Creep, as briefly brought up before. Though, it's much better to frame Xp Creep, as XP Inflation. I think it much better describe the issue. And Mus as they age, in general suffer that. MMO suffer it too with gold. So in general, any mu that has XP needs to implement as many XP sinks as possible. As it's hard to constrain the money supply, in a mu*.
A really cheap, but also crappy solution, is to provide a new game+ tax. Any character which has xp roll over has an additional tax of X%. This'll upset most players, as it's devaluing their time, invest and devaluing their reward.
My current solution, is to allow new game+ characters, to use their XP to purchase limited use Boons. Since WoD is the primary game we're working against, I'll try to provide examples for that. Though I like nWod and oWod, I don't strongly differentiate their mechanics. So please forgive, if I do one then other, or do a combination of both.
Purchase Success Dice. When used, they're counted as additional success. Maybe restrict it, to negating 1's.
Purchase Discounts.
Purchase Inroads to the Game IC world. (Such as having pre established relationships.)
There probably quite a few others you can do.