i liked this game! would make a good mobile app with more stuff like you said "alternate 'character' and skills or something."
however be careful because theres a ketchapp game very similar called zig zag or something.
my recommendations if you want to see profit in the mobile app area is to add those things you listed and:
make people more obvious to see, they look alot like zombies
maybe make it so its not instant death if you hit one, give a debuff or something such as a speed boost making the game harder, a large score decrease or something like that. maybe even a hp bar that suffers blows per human hit and slowly regens per zombie.
if you REALLY want to go above and beyond, up the environment more. Hills, ramps, jumps, walls, explosive barrels, railings you can crash, crates, stuff like that.
some skill ideas:
small ball- easier not to fall off and dodge humans
blades- kill everything on the path regardless of positioning besides humans
gold ball- cant kill humans until its depleted
large track- larger track easier to dodge humans and not fall off
wrinkle in time- slows down your ball for combos and tight spots
hover ball- doesnt fall if it goes off track, you can still stear it back onto the track for a limited time.
raise ground- no more track for a limited time, you can drive the ball anywhere
double points- .... double points
jump- gives option to jump over incoming obstacles such as humans
