I came across this rumour
on reddit, a guy claiming to have attended a private PAX panel hosted by Bioware reveals some info about the next Mass Effect game. People in the know seem to think there's a good chance it's legit.
If true, it looks like the next game will be a sequel, with two new races (possibly indicating it taking place centuries after ME3), a renewed focus on exploration, and multiple types of vehicle. Sounds promising.