You'd have to resprite
Bubblegum (3 directions, currently animated)
Bubblegum's loot (4 items)
Drake (3 directions + dead sprite)
Drake's loot (~4 items?)
Colossus
Legion
(And his loot)
The boss targeting icon
Watcher (3 directions, dead sprite)
Goliath (3 directions, dead sprite, animated tentacles)
legion (3 directions, 3 directions for attack skull, the organ it drops with both used and unused states)
Bone spears, bone axes, bone knives, goliath boats, oars, bone armour, bone helmet, sinew cuffs, etc, most of which have 4 sprites total each (3 onmob sprites + the item sprite)
Tendril Nest
Ash Walker Egg/Nest
Necro chest
~20 items inside the chest that need retheming or respriting
And if you're replacing the terrain -
The floors have 13 icon states, the lava and chasms have weird hacked up sprites that you need 16-32 little squares each for or something
There are also two dozen ruins currently, and you'd need to remap a good portion of them to be more sci-fi themed. Most of these ruins have at least one special item that would need retheming/redoing/respriting.
You'd obviously need to redo the names and text for everything above as well, and probably some of the code as well. And I probably forgot stuff.
I have been meaning to learn to use GIMP. Maybe if i do a bunch of shitty robots people will get motivated to make better ones
And finally, this is obviously subjective, but it'd be nice if we didn't replace something that's cohesively themed and I think pretty (by ss13 standards) with coder sprites just for the sake of doing so.