Hey there! I get a ton of ideas while playing the game, and so decided I must join this wiki. I can only think of two at the momento though:
A buyable item, expensive, to hopefully limit it's use. It would only be 1X1, or 1X3 vertically, and placeable only on a background tile. It would push a square of liquid upwards, and have a knock-on effect with any squares of liquid thereafter. The only problem I forsee is lag in multiplayer, and of course if too many pumps are placed you'll even get water lag in sinlge player. It would add a lot of creative possibilities, if it's at all possible.
The logical extention of the spider which I'm sure is on it's way. Another monster only using zombie/skeleton locomotion would not capture the full horror of the arachnid. I think a new class of beastie is called for, one that can only/also walk on background tiles, and is obstructed by walls/paced tiles. It would call for a new pathfinding routine, but it opens up the possibilities of things like giant ants, wall-hugging mini bosses, and maybe even an original creature or two that can jump from background to foreground.
What do YOU think?