The simplest solution I thought of a couple of years ago (and maybe it's already been mentioned), was to add -regen to a tanker's attacks. Stacking per ability, but not per use so you can't just auto one ability and afk. It would create the same gameplay feedback in a team that Tankers already have when solo. Little to no risk, but reduce reward-over-time because fights take longer. You'll never reach the end of a TF/SF or start fighting a GM and have to walk away. As long as you've built yourself to survive, with few exceptions (namely enemies that can self heal), you will win. I don't know how much that would matter this point with that -regen temp power now available to everyone.
The other idea I had was for every % over the resist cap you go on your highest resist, that gets turned into a damage or recharge buff. For a defense tank, gaining a resist buff significantly increases your survival. For a resist tank, gaining a resist buff would invariably mean boosting your performance in other ways. This would also turn the old crashing T9s (except Elude, which should probably be changed regardless) into potential performance boosters. It would be a considerable buff to Fire Aura (the proto-brute), Ice Armor (which has never been terribly popular), and Stone Armor (which Brute typically hits resist cap on, to say nothing of what Tankers reach). But, even if this was a good idea, I don't know how it would be coded. It may not be possible.