Rosario is a good option. Other young outfielders to consider trading for include Happ, Kepler, Joc Pederson. David Peralta is a little older but would have two years to go before free agency.
There are options out there. But whether they are superior to Markakis depends on price. The price for someone like Rosario might be along the lines of Soroka or Folty. Probably Soroka plus given the uncertainties about his shoulder. If you look at Rosario's WAR numbers they are similar to Folty's. So a trade of those two would be close to fair. Both would have 3 years of contractual control left.
We shouldn't fool ourselves that there are better cheaper options out there. There are good options out there that will not be cheap. There are some cheap options like Mike Joyce. But he is a replacement level player, who really is not a full-time player.
This was pretty much the point on Markakis. There's going to be a lot of sentiment from the players, Snitker, and the Coaches for bringing him back. I haven't seen the first person mention offering him a 3 or 4 year Gordon-like deal. If he'd take a 1 year "swan song" deal (maybe even with a team option) they could do worse, especially if the available upgrades are going to require 4 year deals. As mentioned above, if AA's convinced that Riley is ultimately going to be "the answer" at 3B you'd have the option to turn Johan into an OF (pretty intriguing, actually - even though I'd hate to have his defense wasting away out there).
Given how well Laureano and Canha have performed and the lack of healthy young arms in their system (especially with Manaea and Puk already down for next season), I wonder if there could be some interest in trading for Dustin Fowler. He's struggled pretty mightily against lefties in his first extended taste at this level, but he's still just 23, another Georgia kid, and lit it up while back in Nashville while handling LHPs just fine.