Wasn't Kingdoms Under Fire 2 supposed to be a hybrid of an action MMO and rts? The game that has been delayed for like 7 year then announced last year that they would release in 2015 and has thus went radio silent/dead again?
Extra credits did an episode on MMORTSs back in 2010. They throw out some suggestions at the end about having instanced play with a progressive overworld. The closest game I can think of that is trying something like this is crowfall although that's still much more mmorpg than rts.