So, I don't mind giving a brief write-up of a set of powers that I think might work nicely for what I've got in mind. Though, I did want to first address some of the criticisms that I got.
Claws is the 'quick melee' and 'low endurance cost' power set.
Dark Melee already has to-hit debuffs and Energy Melee already has disorientation.
What would this power set contain that isn't already covered by a mix of other power sets?
First, to address Claws. I'd be fine with a different skin on the powers to make it fit the theme of a speedster powerset, though it's definitely not the only 'quick' and 'low cost' power set when it comes to Scrappers/Brutes/Tankers. Katana is also a pretty quick recharge set that has relatively low costs. And when you start looking at the average and slower power sets they are not shy when it comes to adding in a variety either for functionality purposes or for theme purposes. Adding a new set that actually feels good for a stereotypical speedster fighting style may not be high up on their priority list but I don't think it's something that should invalidate the possibility.
Second, to address feedback about Dark Melee and Energy Melee having to-hit debuffs and disorientation effects I'd like to point out that a number of power sets have these types of effects. It really comes down to how often those effects show up in each power set. I don't think that every single ability in the Speedster power set I'm suggesting should have these effects, I'm just suggesting some possibilities. Things that might make sense for this power set would be something like to-hit debuffs, temporary defense buffs, or disorientation effects. It doesn't need to be the primary theme of the power set.
Third, to address the last main piece of feedback I got regarding what this set would contain that isn't already covered by a mix of other sets.. Right now it is extremely hard to come up with a stereotypical speedster character. You can somewhat pull it off through piecemeal picking and choosing of several different sets, however it can feel a little bit 'janky' to do both on acquisition and on execution. Flurry/Dark Melee/Sands of Mu all feel really good in terms of what an attack might look like for a Speedster but they have a pretty nasty animation lock through a long execution time and they all pretty much look the same with maybe a different coat of paint on each. You also can't enhance Sands of Mu so that very quickly drops off. Most other power sets have abilities that may be able to be considered a speedster power but just don't 'feel' right with how long they take to execute and how long they take to recharge. One thing I'd also like to see that I mentioned previously is a bit of a 'dash' to a target to attack. I think being able to zip between targets like that as a melee character would feel really fun and thematic as a speedster, though I'm not sure on whether it's possible with the tech they have available to them in the game.
Finally, here's a sample of some ideas I've got that might work well as a power set. Though, it's just a shot in the dark as to what I think might work well.
Speed Brawling
Quick Attack - Quick Left Jab - Very Fast Recharge - Rush to Target if outside of Melee Range
One Two - Quick One Two Punch - Fast Recharge - Rush to Target if outside of Melee Range
Furious Assault - Moderate Damage AoE - Disorient - Similar to Flurry
Build Up
Fast and Furious - Quick Right Hook - Self Defense Buff - Rush to Target if outside of Melee Range
Confront/Taunt/Placate
Whirlwind - Similar to Super Speed Power
Siphon Speed - Defense Debuff on Enemies - Defense Buff to Self - Restores Endurance to Self
Infinite Mass Punch - Extreme Damage - Knock Up - Disorient - Rush to Target if outside of Melee Range