I agree more classes would be nice. Even nicer would be new enemies and I think mercenaries would be perfect. That should be low-hanging fruit for the ME3 MP team. I think we're far enough away from release that they could start sprinkling in the goofy stuff like a Biotic Volus and stuff like that. For me, it's all about variety in what we have to do. More than anything, I want new objectives. I was disappointed with the escort objective. I had visions of biotics casting shields to cover everyone or seeing a person run with their hands over their head while you provide cover....just like in the campaign.
And the big one would be boss waves. Not three banshees + three brutes in one wave...I'm talking about what we touched on a hundred pages ago. A harvester for the reapers, a Geth Armature, and maybe freaking starfighter for Cerberus. Add in the Mercs and you can fight against one of those helicopter things from ME2.
As for locations, I'd like to see something on the Citadel (Presidium) and something in a nightclub, I'd also like to see "throwback" maps that would be in one of those cookie cutter merc warehouses or an Ilos map from ME1. From ME2 I'd like to see an Omega slums map or a map on the Shadow Brokers ship/base, which leads me to...
Environmental waves. Just like boss waves, they could have waves on some maps where there is a sandstorm (and snipers get bonus visibility) or thunderstorms (biotics have the edge) and stuff like that.
Lots of potential and I wonder if they're waiting for the fall for a bit MP boost or if they'll save those ideas for a future release.