By "solo" do you mean "free-for-all?" The way Ghost Recon Online works currently, I think a free-for-all mode wouldn't work very well. The core of GRO is tactics and teamwork, and when you're alone, half of the game's core goes away. I'm not saying it's not possible; I'm saying it would require a lot more work than simply adding the mode into the existing game.
As for adding in maps from the classic Ghost Recon games, well, anything's possible. This is a free-to-play game, and as such, you can probably expect to see new content being rolled out fairly frequently. Who's to say a classic Ghost Recon map (or even a free-for-all game mode) couldn't be added?
Edit: Sorry if I'm being redundant. For some reason, warghost819's last post didn't show up until after I posted this. Weird.