Ah yeah you're very right, content is key for sure.
Orcs Must Die is a tower-defense / shoot-em-up hybrid, familiar to this (which we realised after we made the game). They encourage replayability quite brilliantly by allowing the player to go through the maps in campaign mode, then play again on harder difficulties, or try an endless mode. There's also a co-op mode, and leaderboards for each mode / difficulty shared with friends, plus global highscores. Then there's a perk tree system that allows you to gradually unlock more skills & traps as you earn more stars for doing well in the levels.
We're also contemplating introducing a light touch of roguelite elements to randomise powerups in the level - something like a "choose a bonus from these options" kinda thing each night, to encourage the player to try different playstyles (e.g. you could play aggressively and focus on possession, play strategically and lay more traps, or focus more on stealth and try to thwart their plans without getting caught). But this definitely requires finetuning.
There's also a reasonably low-hanging fruit of local multiplayer. But on Steam that mightn't count for much (more on console). Online MP is out of scope for the first release, I'd say.
But, assuming we created ~10-15 hours of replayable content, it sounds like you're saying Phantom of the Grove's gameplay is interesting enough and the premise is captivating enough that, sans the bugs, you'd give it a try for $20 USD?