Already planned, though obviously not yet implemented.

Personally, I'm less sure that resistances need to be changed to a rating, but not because ratings are inherently inferior or anything like that (we do, after all, use ratings for evasion and armor, albeit different sorts).

Rather, I think it's good to have different mechanics for different defenses - yes, to the point of those increasing returns. And it's also quite easy to neglect them (and then regret it) due to the relatively intermittent way resistance gear shows up (even if it is easy to find, every resist mod takes the place of another, after all), or even to lose them (especially before endgame, which is where I for one spend the vast majority of my time).

Also, from a development perspective, there's probably an upper limit on the number of different mod names you can come up with that sound good and mean "resist fire better", especially if you're trying to make each sound better than the last.

That's not to say I think a resistance rating is an abomination-that-should-never-be-visited-upon-the-game or anything like that, though. I'd expect if such were implemented, we'd also see multiple tiers of resistance rings, and a line in the character sheet detailing just how much of X effective resist we had against the last monster we fought.