I believe they’ve said before in interviews that Classic dungeons just aren’t compatible with the M+ format. It would take way too much redesigning of the dungeon layout and boss encounters. Especially since a lot of the Classic bosses are really simple fights with just a single mechanic.
As much as I would love to see it, I don’t think they’ll do anything older than Wrath, maaaaaybe TBC.
For some of them the only true mechanic that would be relevent as is is the timer, the routes wouldn’t be just pack pulls, but trying to save every little second you can walking between stuff.
Old dungeons were made to be fun and felt like real dungeons. They aren’t compatible with the zerg dungeons today.
Dungeons older than Cataclysm don’t have much in the way of mechanics.
It would be cool to see the logic basically applied to all instances but maybe the older stuff just can not be converted.