Making games is hard. For me, one of the harder parts is the initial idea. I'm rather slow to generate new ideas.
Of course, there's some tricks you can employ to make the process a bit fasater, and there were tricks that i employed.
For me, what i like to do, is think of an existing game that i find fun and enjoyable, then deconstruct that game into it's component mechanical components. Then, with those components, figure out what you like, what is fun, and what is engaging - easier said than done.
one classical thing to do is a fusion of games that you enjoy, but one must take care to not draw too heavily from the source as to not be original, but also to keep your ideas in check to not be too outlandish to work. It's a delicate balance, not to mention some games will be more compatible with the other.
the next step is to then make it your own game. put your own spins on it, had a good idea? write it down! this will further help differentiate you from the rest, an especilly important thing to do in the saturated market we have today.
as for how things were differentiated, let's take a look at noita. in noita, spells go in wands, and those wands have various stats, dynamic and static. Mana, mana max, mana recharce, cast delay, recharge, capacity.
there's also the gemcaft inspirations to talk about. a beloved franchise of mine that i remember playing with flash (07) a lot. in both games, there's gems, gems go in towers, gems can be fused.
but that's where the differences emerge.
gemcaft has unique attack bonuses on gem types, fusing gems merges those while also making them more powerful, additionally, there's multiple tower types. amplifiers cannot attack but boost adjacent towers, and traps, which are faster and proc their bonuses more, but are weaker in damage.
i decided not to replicate many of the mechanics to preserve identity, but also for simplicity. as much as i love gimmicks, they can go overboard. sometimes you just want a simple game that does 1 thing and does it well.
and of course, there's other nuances of differences that could be talked about, like pathing, or spell casting mechanics, but i'll tackle that later.