I was thinking maybe there could be some new upgrades to buy from the seller, these could include; 

Vampirism: you have a 10%  chance of gaining a life after killing an enemy.

Double jump: does what it says on the tin.

Lucky: 20% more likely to trigger an event like a gun shop.

Aura: you create an aura around you that deals damage and knocks enemies back.

Medkit: once or twice per run the player could restore some health.

Turret: you can place down a sentry turret that shoots slow-moving bullets at enemies, it can take damage but it can be collected by the player and will restore health slowly while in the player's inventory.

If the player got so many abilities there is a risk of getting overpowered so you could introduce a limit on how many they can equip per run

You could also upgrade some of the abilities you already have like 3 more life or wall slide becomes wall climbing.

There are so many ideas that this game could use and I'm excited to see where this game is headed

I will continue to post new ideas when they come to me

Illusion: you create a hologram version of yourself that enemies will attack/fly at if it is closer to them 

Hey! Thank you so much for a lot of ideas! :D

Just to let you know that Double Jump is done. It will be available on next update ;)

Thanks!

Barricade: you place a wall impassable to most enemies (you could create an enemy that phases through terrain), so they either have to destroy it or find another way around. This could be used to make artificial choke points to maximise the use of weapons like the laser or flamethrower.