Well hasn't it been said that the next Mass Effect game isn't even officially called Mass Effect 4?
If there's no ' 4 ' at the end then I'd find it hard to believe that it's a sequel anyway. They wouldn't need a ' 4 ' for the next Mass Effect to be a sequel and I know that but how would you call it then for it to be a sequel without a ' 4 ' when in fact ME2 and ME3 were numbered and had no subtitles. My point is that I don't think that the "Next Mass Effect" will even be a sequel to start with. I just have the very bad feeling that the game's events will indeed occur during the same events of the trilogy, be it during ME1's, 2's or 3's, exactly when during the trilogy we don't know but it's supposedly been "hinted" already.
At this point in time, however, it's probably too late for BioWare to change that, specifically, they ARE working on it already it's not like they still need to plan the very basics, they're past the "let's try to figure out just what ME4 can be" phase. They surely planned something already and I do think that it will be during the trilogy's events sometime between ME1 and ME3. This is ultimately just speculation overall but it isn't now that BioWare are going to go "You know what, actually, let's scrap the plan we have right now and let's make a reboot instead even if it means starting all over again and releasing the game in a century from now". Nope, won't happen.
But, keep in mind that I completely agree, a reboot is a necessity for the franchise, in my opinion as well... it just won't happen, put on X on that.