Player created powers were a popular idea on the official forums back in the day & I would still love to see the idea be fulfilled.
The way I imagine it working is at first players would only be allowed to create attacks as those are easier from a balance perspective, more fun & limiting the initial scale of such a system would making easier to develop & result in quicker delivery.
Ideally you would be able to create 1 ranged attack + 1 melee attack & using a point system to choose traits like the dmg type, range, inherent accuracy, autohit, secondary effects, 0 degree cone & end cost.
As for visuals, I think the player should be given wide latitude to use any elements of preexisting attack's if those elements are already decoupled from each other like:
-Animation & sounds ex. knock out blow
-Special effects ex. Em's pom poms of doom on one or both fists
-Hit visual effect ex. Crushing Uppercut
-Projectiles if applicable ex. Rune of Warding
-Summonable Pseudo pet to launch the attack from if desired
along with a warm charge up &/or cool down animation selected from emotes like Kata, buffs like armors etc
Since you wouldn't be able to slot these powers I think they should be powerful from the get go while also accepting Alpha Slot, Interface Slot & global bonuses ignoring Fury and possibly recharge aiming for a 30 sec recharge on said power.
*I have no idea what is or isn't decoupled & if these things aren't decoupled already it would probably be too difficult (or impossible) for the homecoming crew to decouple them so that they can be grabbed a la cart