Nice.
Leaves me wanting an upgrade screen between levels and different levels, sunk so many hours into little browser games that work that way over the years!
But the timing is fun.
Because of the delay and pathing of weapons, you could totally have different types of weapons that function differently, i.e. have some that arc, some that fly straight, or a beam that hits the target instantly, things with explosive damage.
You could make it a little grindy if you wanted to. Usually games that have a tiny bit of grinding end up being a lot of fun (i.e. say you're a really good player who learned the game well, then min/maxing would allow you not to have to grind).
Actually, that's how I would do it. Put in upgrades/resource system, some grinding, some bosses and then a leaderboard for completion with less restarts.
After that I'd say it would be a pretty sellable game. Some of the other mechanics mentioned, like different paths would make sense. Also, wizard guys could attack from a distance or shield enemies, weapons could deal with that.
Make a matrix in a spreadsheet with monster abilities on one axis and player upgrades on the other so you can map out the interactions and make a plan to keep the scope of adding those features relatively manageable.