This is probably a joke post but I've been contemplating it for a while with all the doomsday talk going around and it could work. If SG is looking to grow DotA 2 is probably the best way for it to do so. It's been the top game played on Steam for a while now and almost always has at least 100k current players. Obviously there's the matter of Valve running all the servers but if SG did something similar to D2ware where you guys made your own custom servers with custom plugins it could work. I play D2ware pretty regularly and there's almost always at least a couple lobbies going on with at least 200 or so current players (and I usually play pretty late at night). This would require people who know their way around Sourcemod though to create the plugins and manage them. You guys could maybe even get affiliated with D2ware as currently they're just a service, not really a community. They have no type of forum or ways of communicating or getting together other than a singular email address that belongs to the developers. Obviously it wouldn't be simple or easy to do, but it's worth at least mentioning I think.
The DotA 2 map editor is rumored to be coming out soon as well and with it most likely means third party servers. If SG were to be one of the first to run a server running favored maps such as Pudge wars, I think it could be huge. Old school WC3 players have been dying for the map editor to be released so they could play custom content officially, as many are either unaware of D2ware or simply refuse to use it. I'm almost 100% certain that as soon as the map editor gets released that the first couple of servers to spring up will be full almost constantly.
Another thing I'd love to see come back is a TF2 server. I know that in the past it hasn't worked out, but you can't neglect the numbers TF2 still manages to pull on a daily basis. Despite being over 6 years old it still manages over 50k players a day, and is most often at least top 3 on Steam. I think a different approach this time could result in a successful TF2 server. Last time iirc SG had a 24/7 payload server, but I believe if we tried a fun server (aka minigames server) it could work out nicely. If you check gametracker, you'll find that there's not too many dedicated fun servers out there. If SG ran one with a good map rotation and good plugins, I think it could be much more successful than another one of the million other payload servers. If it does come to pass that SG does start up another TF2 server, regardless of what type it is, I promise to try and help populate it as I recently came back to TF2 and still have some friends that play regularly.