Moving a splash piece onto an adjacent square currently does not kill cornered/against-the-wall goblins, even though they cannot "run away". (Ex. moving to B1 with horizontal/vertical splash when goblin is A1)
Blightsack tooltip says that it leaves blight on all adjacent squares upon death, but only leaves it in diagonal squares.
(I would personally rename blight to toxin/scourge/plague, as having blight matron, blightsack, and blight(power) can be confusing to refer to). Also I think you could experiment with having the blightsack death effect only last one turn rather than be permanent.
I like your idea of having hard unlockable dungeons. These could give permanent upgrades to a "campaign deck" that you have.
Imagine a campaign where you start with a completely vanilla deck, and continue with the same coin/upgrade system as in this game, but is initially too difficult to beat all floors/worlds. Once you lose, you have to start the campaign from the beginning. By doing hard side dungeons, you can unlock bonuses to this deck whereupon restarting the campaign, your deck is stronger to eventually make it to the end of the campaign/world.
start campaign with +1 heart (passive)
all power-ups cost 1 less coin (passive)
+1 coin per floor (passive)
start campaign with an additional splash/shield/cantrip power on a piece (passive)
These bonuses to the campaign deck can either be "beat this hard side dungeon and get permanently get this power-up", or could be "you beat 7 floors on this run of the campaign, you get 7 shop tokens to spend in the upgrade store tab".
The campaign over world could be similar to Mario, and once you reach the end of the world, you receive a checkpoint and the next loss restarts the campaign at lvl 2-1 instead of lvl 1-1.
Just spit balling ideas here, easier to imagine than to implement. Not expecting anything of the sorts!