I think the Mega dungeons are already filling that challenge really. They are hard dungeons when they come out, you get a great experience taking them on for the first 5 times and then you can leave them alone.
I think if you are really looking for a hard challenge in dungeons, where there is no timer......Then just time out a really high key If can't beat a +13 timer, then you won't really have the stats to beat a +15 without a timer. The bosses often seem to me as the deciding factor in a M+ run. Also, the timer is not really anything other than a sudo-enrage for the bosses in that dungeon. Can't beat the dungeon in the appointed time? Then the bosses would have enraged if you had tried to do it without a timer.
So im really curious where OP, and anybody interested in his idea, sees something missing from the current Mega Dungeon and M+ experience, especially when you count in just having timers run out on high M+.