I first started playing when CoV released and always leaned more red side in terms of preference, but generally found more game time blue side.
I think part of it was to do population/availability of teams. The other was there was just less content overall so there is the same natural amount of user engagement (but overall I'd say red side content is better quality).
I think there's lots of potential options, but being pragmatic I think going after shorter term goals is better initially than trying to go after something as a big as a new zone (which would be cool but not small undertaking).
Some ideas from me:
1. I think I'd try and make Mercy a bit more of a natural hub similar to Alta's Plaza. Atlas Park is always full of folk from multiple levels whereas red side it's generally silo'd into level appropriate zones. Red side needs it's 'hub' and for me Mercy fits better as, like Altas, it's accessible level 1. Adding a second black helicopter that's actually a contact launching 'raid' style mission arcs across multiple level ranges could be one idea. E.g. starting with pushing Longbow back and eventually striking at Paragon City locations, tiers of the arc unlocked as you level creating a natural flow of traffic in the zone.
2. Add another TF in the mid level range, again I'd likely make this a contact in Mercy, maybe by the Widows tower or something.
3. Red needs more GMs, GM hunts are popular blue side but there's only a handful of red. Lore wise I think there's scope to build on existing models, e.g. add a Wailer GM to St Martial as some kind of event where it's trying to attack the Giza.
4. Reds would benefit from some more zone events like the Hellion Arson in Steel Canyon. Maybe something like a Snakes outbreak in Mercy/Oakes and Mongoose sends you off to blow up the nests they're popping up from or something.
5. In terms of a 'new' zone, I'd consider opening Croatoa up to reds as a co op zone. It's Lore kind of fits into it's own box, could be reached via sub, opens up the Katie H TF and adds 2 more GMs into the list reds can reach.