Players could make their own challenge, with starting mods, +/- percentages to stats, pre-selected weapons, shields, and ships, enemy stats, wave progression, the ability to level up or not, ect. After they've made it, they could upload it with a code given to them, so they could share the code and have other people play their challenges.
for example, a Laser gun challenge could have:
+30% fire rate
+50% rotation speed
double enemy spawns
half enemy health
+10% enemy movement speed
If you've ever played BTD6, they have a thing like this where you can make very specific challenges for other people to play with given codes.