Certain abilities probably wouldn't be too difficult to make. Hermit Purple would probably just work like Bandages' tether, Hierophant Green would probably just be projectile spam, Magician's Red would probably just shoot a fireball, Crazy Diamond would probably just give rings to friendly players, The Hand would just pull enemies closer and hit them if they are already close, Sex Pistols would probably just shoot homing shots, etc.
Others on the other hand might be more complicated, such as Killer Queen, Echoes, Sticky Fingers, Moody Blues, King Crimson, etc. I'm not even really sure what Hamon could even be aside from a stat buff or something.
Overall the most intimidating thing is probably the prospect of how many wildly different abilities there are. Make one, and people will start whining to have their favorite character's abilities added in. If whoever was working on it wanted any hope of pleasing everyone, they'd have to make it into a large scale ability pack, which would take a really long time, a lot of creativity, and a ton of coding skill to make.