I would absolutely love to see your first suggestion regarding $regsub added.
I have used $regsub a lot, in cases where knowing how many substitutions there were (what $regsub returns), and in cases where I only cared about the substituted result, and to be honest, 90% of the cases is the latter case, for me and most people.
$regsub dynamically re-evaluating the substitution part would also be a nice new feature, something that's been suggested a couple of times before I think.
