Instead of f2p questing script, an actual account builder would be pretty epic, so start the script straight after tutorial, and let the script do a multitude of skills and quests until any desired point. e.g. dragon slayer from scratch just by botting.
Aside from that, you have castle wars which is f2p, runecrafting, collecting items (picking potatoes, garlic, etc).
And if none of these sound good, you can always re-invent the wheel and make the wheel better + faster, something which nobody is doing; e.g. mining, there is still no 3 tick mining script out there.
Good luck!