Glad to hear this, I think the kind of instances used probably depends on how many dungeons are planned. If the plan is to introduce a final challenge dungeon that will take a long time to complete then an equally long and grindy dungeon like BRD would fit nicely. If the plan is to add multiple instances I think more similar to what we have would be fine, just gradually being grindier I just think the risk of power creep is a bit higher with multiple smaller grinds.
@Moonkinstar recommended Scarlet Monastery and I think it could make for a fun dungeon to farm too, with the different wings and bosses dropping different materials so you can't just farm the same areas over and to give reason to clear the whole instance.
I think that's probably the biggest weakness of the current challenge dungeons, there is no requirement to clear the whole thing to get BiS upgrades, for example, a popular method for the current final shadowsbane upgrade is to just do up to Al'ar and then reset, meaning most people never get to fight the quite challenging boss that is Kael'thas as it's not 'worth' it to do.
I think along side upgrade materials, ultra rare drops like mounts could be added to give additional reasons to farm, farmable titles, etc.