At least for me, the change that would offer the greatest impact would be to proliferate as many NPC costume pieces as possible, including weapons, and to allow more crossover of weapon models between various powersets, like getting Arabian Scimitars into Dual Blades (even if the perceived theme doesn't always neatly fit the damage type, such as allowing D.U.S.T Rifles or Resistance Energy rifles in Assault Rifle or Arachnos Soldier options).
I remember when Icon went live, there were a lot of NPC costume parts that became available for use. I know that got reverted for some technical reason, but I would really like to get them back.
EDIT: It would also be nice to have costume pieces only currently associated with specific powers made available, like many of the Flight Packs. I have some costumes that look especially good with the LKT-1700 for instance, but don't want to hemorrhage Inf just to maintain a costume piece, or have to always be flying to have it on my back...