I know this may have been suggested before but given the reason and storyline I am suggesting it may work. Read on....
In a lot of comics I have read, a well established hero or villain always seems to get reinvented, killed off, changed by an ever expanding multiverse etc. The main theme seems to be an alternate version that has developed differently or a time changing event that alters the character. What I am leading to is a FULL respec - including changing powersets.
Before this gets shot down though please read how this would work (at least suggested to work).
Prerequisite of Lvl 50 and Oroboros Portal.
Would be a TF with arc leading up to it.
The arc would deal with somebody messing around with oroboros crystals to change the past and stop them getting stopped in their attempt at global domination. The arc follows you foiling them at each stage until you finally catch them - or so you think - right at the end when they are defeated they go back in time in an effort to kill you before you get your powers (cue a lot of sci-fi movies).
You manage to jump back to the same point slightly after the villain has made their escape but in coming this far back you are exemp'd to 1st level. You are also now on a string of missions stopping said enemy from destroying you by stopping the event that gives you your powers (mish will be different dependant on origin)
Once completed you are transported back to the oroboros area but in going back in time you have changed history and your life has played out slightly different - you get a FULL respec including powersets and can't return to paragon until you have chosen. Only the mish holder gets the full respec - other teams members would have the option on a normal respec. Both choices given in same style as terra volta so respec still optional.
I personally think this would be a great idea and wouldn't harm the game play whatsoever. The FULL respec could only be granted once to a lvl50. It has to be a lvl50 as the effort to get to lvl50 with the powersets chosen to start with needs to be the same for everybody.
Now you have read it in full - let me know what you think - would love to see a Red Name comment