Easiest, make generating orbs an intrinsic perk on exotic weapons. Second easiest, put a masterwork slot in exotic weapons and disconnect orbs from catalysts... you know, just like almost every other weapon in the game. Has the benefit of using existing systems. Has the bonus of giving you flexibility to create future functionality. Third easiest, add a weapon mod slot to all exotic weapons to make them consistent with like almost every other weapon in the game. Then create a mod that "Generates Orbs on Multikills". Only make it compatible with exotics. It'll do nothing if you already have a catalyst. Could also just release an "Exotic Spec" mod that does nothing on non-exotics like trying to use Dragonfly Spec on a weapon without Dragonfly. Has the benefit of using existing systems. Has the bonus of giving you flexibility to create future mods. Fourth easiest, unlock the catalyst slot on all exotics, and either default it to a placeholder catalyst that just generates orbs, or let it accept something like a weapon mod (as above) that generates orbs. Has the benefit of using existing systems with minor modifications. Hardest, rework a system that's been in place for years (What could go wrong?!), break all the existing builds, and make players hate you.