The three primary issues I've personally found with Nexus Blitz:

These are just the three biggest things that I feel negatively impact the gamemode. I personally feel that fixing these things will likely fix many of the issues it has. I won't claim to be an expert, but these are all based on things that should work out overall.

1: The double damage melee champions deal to melee and cannon minions affect lifesteal the champions have. This means that if you have 100 AD and 10% lifesteal, and you hit a melee minion, you'll deal 200 damage and recover 20 hp. This causes problems with champion with innate lifesteal, especially Nasus, as it makes them nearly impossible to poke down. Fixing this so that lifesteal, spellvamp, and all other, similar effects apply pre-doubling, rather than post-doubling, would make them much more bearable.

2: The on-fire buff makes certain champions nearly impossible to fight. Ziggs, Vayne, Kayle, or Nasus got the buff? Game's over, you might as well surrender. It needs to be toned down a bit to make it possible to fight them. The damage bonus is a bit too high, and the CDR it gives is too strong. Nerf it by reducing these numbers, not by increasing the damage they take. While it'd work to fix that for those champions, it'd hurt full-on tanks who got it more.

3: The queue has no music. It's silent while you're in the Nexus Blitz queue. This makes it feel like it takes much longer than it actually does, especially without people in it. Just put the SR queue music in for now as a placeholder if you intend to give it music of its own later, just so that there's something there.