UPDATE: BioWare's Mike Gamble has confirmed that these words were not his own, and the original interview from which we drew this information was fabricated. We apologize for publishing this misleading information. The original story follows.
The next Mass Effect game has a rough release window of late 2014 to mid 2015. Mass Effect 3 producer Mike Gamble told Gamer Syndrome (via Eurogamer) that full development started "a month or two ago," and we can expect to hear more in 2013.
Gamble also spoke about a tossed ending idea for Mass Effect 3, one which some fans have complained the game lacks--watch out for spoilers in the next paragraph if you haven't finished it yet but still intend to do so.
"We also scrapped the original ending for Mass Effect 3," Gamble said. "We were gonna have it as a boss battle, where Commander Shepard would go up and fight the Illusive Man who has been transformed by The Reapers. We scrapped that idea because way too many [games] have that as their ending, some final boss fight."
BioWare initially hoped for Mass Effect 3 players to visit and explore Palaven, the Turian homeworld, but the final campaign included only a detour to its Reaper-sieged moon. Gamble said the game's co-op multiplayer--which was developed at BioWare Montreal, the main studio behind the next Mass Effect--was also first designed for Mass Effect 2, though there wasn't time to implement it.
Gamble strongly hinted that Wii U Mass Effect 3: Special Edition players may see some of the game's DLC coming their way, though the company has nothing to announce at the moment.
Mass Effect executive producer Casey Hudson previously asked fans whether they would like the next game to be a sequel or prequel, and noted significant interest in playable non-human protagonists. It looks like we'll find out sometime next year if and how their feedback has impacted development.