I mean, throwing in a few dungeons from previous expansions as a M+ dungeon would be fantastic. They can either retool the dungeons to make the story fit within the confines of the current expansion, or just literally have it be a blast to the past and make zero sense as to why we're doing it. I think retooling the dungeon somewhat to make it fit the theme of the expansion would be the best though.
Assets, loot and overall dungeon design is already done. All these dungeons would really need it a pass over balance wise and 'lore' reasons as to why we're there again. Speaking honestly, most dungeons prior to WoTLK would need massive overhauls and require a lot of work to fit with how dungeons are designed these days. A lot of Vanilla/TBC are far too simplistic in design, with most bosses and dungeon trashes main 'difficulty being 'hurr durr, I hit hard and apply random disarms, mortal strikes and miscellaneous CC'. WoTLK had easy dungeons but a lot of the bosses actually had mechanics, meaning that all that would need to be done is tuning.