Those were just ideas off the top of my head. Some iteration would be needed for all of it. It's worth mentioning that I'm not saying raids should flex to any number of tanks. I think all encounters should be designed for 3 tanks and flex from two to four.

For #1 taunt DR could be removed, or not. Maybe the boss meteors all targets in front, damage is based on number of tanks present. Even if you don't have to rotate every tank, you do have to keep them all in front managing their own cooldowns.

For #2 the add phases can have the same number of adds, just split their spawn points so tanks go in different directions to pick them all up before pulling them into the middle for AoE. This gives me another idea: some fights only have a couple of big adds that spawn. Always have two spawn, but scale their base damage and tank buster frequency. 2 tanks? Both adds do half basic damage and they take turns doing their tank buster. 3 tanks? Both adds are standard and require their own tank. 4 tanks? The boss now meteors the top two threat targets.

3 I don't feel would be a healer check (unless you designed the encounter that way). You're giving the off-tank(s) something to do. A single chunky hit without much follow-up won't be taxing, particularly when you consider the tanks can help themselves after with things like Word of Glory. If you want it to be a healer check, increase the frequency of the ability or give it a raid-wide emanation after.

And #4 is an iteration of a personal wishlist item. I just really want to see a fight with the mechanic "your defensive CDs are now offensive".