Mass Effect can't be an RTS because of the established backstory. The majority of the aliens are at peace with each other, or at least have no interest in blowing each other up. You'd have to come up with an explanation for all these races suddenly deciding to wage war on everything in the galaxy. This isn't Warhammer 40,000.
That being said, if by some miracle they actually created a good explanation for everyone going kill-crazy, then I'd be all for it. Once you get rid of those pesky alliances there really isn't any reason Mass Effect couldn't be an RTS. The potentially certainly exists. But of course, the backstory to Mass Effect is actually important so we can't just butcher it to make an RTS.
The only type of Mass Effect I can imagine ever being made is one similar to Warhammer 40,000: Dawn of War 2's single player campaign. You manage a small squad of soldiers as they lead a campaign against the criminal scum of the terminus system. But that certainly wouldn't involve the big epic battles most people would imagine. Killing Batarian slavers wouldn't be nearly as satisfying as, say, leading an army of Geth into a Quarian stronghold.