When my old main finally dies I generally come up with a few character ideas (I have way more than I can ever use anyway).

I start up the three or four I like the most and play them into the mid 20's or 30's (depending on when they get what I consider to be their defining skill/style).

At that point I go with the character I feel to most connection to and have the most fun playing (not always or even normally the one I pk with bet, although never one I flat out suck with).

Delete the other characters or let them auto, and the one left standing is now serious.