Full Disclosure: I have little experience with the Interceptor but my idea may still help make melee more fun for them too. I know there are a couple of other posts on Melee but I feel this is different.
-----ADD COMBOS----- Not unlimited as the Interceptor already has, I don't want to move into it's role, but make chians available, atleast in the Ranger.
Ranger has components that play to melee but all the Ranger can do is one swing. ONE crudey swing!
Give the Ranger atleast a 2 or 3 swing combo that can also chain elemental effects for example. Let the Ranger's 3rd swing provide a effect chain and when its on cool down allow for a 2 swing combo. That wouldn't make the Ranger compete for the Interceptor's Melee Mastery but will make it less stiff and unimpressive.
Or make it a baton swing with a shoulder bash, something to liven it up a bit.
Storm: Give the Storm a small combo to lift it up or back to play into the Storms' flight and hover role. After its spinny melee attack, allow the Storm to aim a pulse downward, blasting enemies away and propelling 10 or so feet into the air so the Storm can transition to hovering. That would be impressive and fun to play! (and Techno Wizardy)
OR after the Storm's initial melee have it spread its arms out and push enemies away with a force pulse. This wouldn't have to do damage and it could be on cooldown.
Colossus: I really so no reason to change Collosi attack as its does fine IMO. Its a slow, heavy hitter, aoe effect. It works fine I think. Maybe sell us an alternate animation where it stomps the ground with its foot. NO with BOTH FEET LMAO! (I Main a Collosus :D )
Interceptor: I really think maybe changing up the animations would be fine. My limited experience with it and friends who play them tell me of no issues in the melee department.