After resolving any surprise attack, it goes in modified Dex order. First round only, you can opt for triple your weapon length in place of your Dex, if you want. Spell or mental activity uses higher of Int or Dex.Except that, ever in search of ways to simplify, I think now I'd drop the second paragraph, instead using weapon length/spell level as a tie-breaker.
On rounds other than the first, add opponent's weapon length to your Dex if your weapon is shorter. Spell level counts as weapon length.
"Initiative" is, of course, two entirely different concepts shoved together in an ungainly way: play order and action order. My rule covers action order. For play order -- what the "roll a d6 for each side" originally represented -- I would just let the players choose to go first or last, unless they are surprised. Can't really see a reason to worry about it more than that... After all, the GM already has a significant advantage over the players.