I need a rule clarification. When midnight strikes, do you always adda doom token then additionally do what the mythos card says; or do you only do what the mythos card says. I was doing the former, then played Omens and noticed the difference. Now I can't find in the rule book where it says to add one. So I think I've been playing it wrong.

You should only ever add Doom when an effect tells you to. Midnight is not an effect.

That said, if the rule is fun, keep it.

I agree with the TurnipKing, adding a Doom Token regardless of what the Mythos card says, will definitely add some excitement to a game in which win ratios are much, much higher than necessary.