So if you loved LoL, why don't you play LoL? There are not a huge number of online strategy games for free.
You could check out Age of Empires Online if it's still going...
Yeah literally, it's narrowed down to ONLY Age Of Empires Online. Because everything else is browser-based casual games using time and "energy" to limit you from playing.
Age Of Empires Online is (or was, not sure) a pretty awesome game. I even put money to get the pro greek civilization and skirmish. But you can go pretty well without it. Support for it stopped a while back, and I'm not sure if it is completely shut down. I stopped playing because it felt abandoned, and really it was. But its definitely a treat for a long time for any RTS fan.