When I have a thought about how some piece of code could/should be improved, but it's not urgent, instead of

  // TODO: Refactor this by doing X, Y, Z

I'll say

  // Hmm: This seems brittle.  We might want to X, Y, Z this such that W.

My IDE will list all the TODOs and I don't like to clutter that list with stuff that isn't strictly necessary, but it is nice to have some string--"Hmm:", in this case--that I can grep for or recognize as indicating that I thought about this already.