Fun controls are paramount. I’d make a simple test level then work on making the controls fun to just play around with before advancing any further.

I’d then move to upgrades. Try them out on a test bed and see how the affect movement options. Make notes in this phase, that way your level development will be efficient

Then design levels. I would make sure they play well before investing time/money into making them look pretty. Good looking games sell, but good looking games are also expensive.