Maybe one can write about using slime as industrial revolution era engine lubricant that cause the whole factory covered in slimy gunk ?
It sounds like another slapstick episode where protagonist's experiment cause trouble to everyone.
Or using slime as underster travel method ? It's basically talking vehicle all-over again.
Maybe Slime Magical Girl ?
We've got the Slime lady and Slime man as the villain since olden days, as drawing slimy monster with tentacles is fast, easy and get the message across instantly.
Or world of slimes where the inhabitants we're slime ? Any human reincarnated or summoned there will be swallowed instantly, not that interesting of a story.
Slime-like powers ? Already done multiple times, most of them were forgotten and only two or three got popular.
At this point, Being "Transmigrated with water gun that could shoot OP Holy water depends on MC's mana" might be more interesting than slime. Especially the part when Transmigrated MC cause Tsunami since MC's Mana is infinite (protagonist buff), a pissed off Mary Sue stop MC, MC died, the watergun taken by another interesting mary sue MC, causing MC swap, and the new MC does interesting experiment shit with watergun.
But the old transmigrated MC lives on as the spirit of the water gun ? It takes care of the problem on how transmigrated MC would adapt and survive in the new world; they didn't as they got killed by locals and now their soul trapped inside watergun.
Or just put the Water Gun into sentience ? As-in Water Gun MC ? Hey, another new refurbished idea !!