The problem is a universal one with D&D 5e and is centered around "power creep" or more specifically - you just killed a massive Netherbrain with the ability to kill entire worlds, so where do you go from here?
Past level 12 the power levels are just insane, especially with spells in the 7-9th level range being pretty Godlike to the point that any limitation on the characters can be overcome with magic.
They could do a prequel, where you start at level one. It tells the story of what happened before everybody got sucked up by a mindflayer ship's tractor beam. The story already makes it clear/plausible that protagonists already had some levels under their belts before they were abducted and underwent a level wipe. So an expansion could go from level one to four/five-ish.
