I doubt they will do a prequel. A prequel would kill the franchise. There would be a lot of really pissed of fans again who were careful not to buy the game on release and will never buy it.
I think the whole ME3 business was because they didn't have enough time and the endings were just thrown in by the main guys without running the idea through other people. this time, this time they should have listened. After all I doubt they want a repeat of what already happened.
And if they do any kind of prequel or sideqeul bullshit then it will end up killing them.
But that is assuming that those two forces are exclusive and independent of one another.
ME3 also "really pissed off fans" because of it's rushed and flawed ending. But the reason that ending was rushed and flawed wasn't because the devs were eager to use up their skymiles and go on vacation. It's because of corporate pressure from above. Regularly released products from flagship IPs is one of the single most important goals for the cashmen.
It has been proven multiple times that quality gamemaking will be sacrificed at the altar of deadlines. Dragon Age 2 is one of the primary examples of this.
Given the broad influence of the decisions made at the end of ME3, there would be a lot of work to make a sequel. Could a successful sequel be made? Absolutely. But it would require easily 2x the assets and development budget as making a prequel or concurrent story. The same internal forces that rushed DA2 and rushed ME3 are the same forces at play in this scenario.
Objectively, "pissing off the fans" will result in depressed sales, but chances are the people holding the cash figure that a critically mixed title released in 2015 with less development costs will make them more money than a critically acclaimed title released in 2017 with double the development costs. In addition, very few video game franchises remain extremely valuable beyond their third or fourth edition. The big ones you can think of are almost all cinematic in focus with almost no "choice & consequence" elements that complicate writing and drive up costs.
I really, really want a sequel. A good sequel. But I also wanted the Shepard trilogy to end with a narratively coherent, satisfying ending. I've learned to temper my expectations.