That's my main concern. Would be so lazy. I want a "What's going to happen?" story, not a "How did we get here?" story.
Trying to think of the series that started off great but now all it is, is prequels...
There are far more comments regarding the potential of it being a prequel (that is, being set before the original) so apologies for selecting you two as examples, but it makes absolutely no sense to me how that would be significantly relevant on the quality of the game. The only difference that may occur between a sequel, and a prequel, would be the presence of humanity, and a 'cap' on the level of technological progress. If it is set before the first game there is absolutely no reason it would need to be a story regarding 'how did we get here'/involve the Reapers at all. How many cycles have occured before the first game? As far as I recall it's not specified but given the amount of Reapers (I know there would have likely been a significant amount from the original creation of the Reapers) we can occur it's a very large amount. I imagine, somewhere, in those hundreds of thousands of years, there is an interesting story to tell. We know that the species involved will eventually become extinct, sure, but that doesn't make the story any less relevant (unless you think all of the species after Mass Effect 3 live happily and nothing ever threatens the existence of humans, the Drell, the Asari, etc.), it's just smaller in nature (and the story is going to be smaller in nature; it's hard to go larger than an ingalactic war between a universe-sized invasion force intent on the destruction of nearly all organic life). The one constraint this has is that the technology cannot be vastly more advanced than what we already witnessed, but that's not to say it cannot be unique regardless (take the Protheans' technology as an example; it was very different to what existed in the current timeframe) depending on how they adapt to the Reaper technology.
Of course, taking it place afterwards makes sense also, there is a large imeframe that they can make use of; from the rebuilding of the galaxy (if they want to go large scale they could introduce a militant species creating their own faster than light travel and taking advantage of the ruined civilisation as they try to rapidly expand), to the establishment of the new, and now much more technologically advanced, galactic order, to hundreds of thousands of years in the future.
Alternatively they can make it occur simultaneously with the events of the original and drastically reduce the scale so that the effect of the Reaper invasion is not really significant to the story being told (that is, it's so focused that the potential Reaper invasion never even impacts the story such as if it were to be about a bounty hunter/SPECTRE tracking a drug lord across the universe in a small time period example) but I don't imagine this will occur.
The only real difference going backwards or forward makes is the presence of humanity (they can afford to do that going forward, not so much going backwards) and other Citadel races (depending on how far back they go of course), the advancement of technology (going backwards, they can't be so advanced that it's unbelievable that they would eventually be conquered by the Reapers; going forward they can be advanced as the writers wish), and to a lesser extent, the reliant upon Reaper technology (if they go backwards, the series' staple items [Mass Relays and the Citadel for example] will likely be present and as important as they were in the original trilogy, if they go forward this isn't necessarily the case [as the species may become so advanced they are outdated]).
Maybe I'm missing something, but I don't understand the 'oh no, don't make it a prequel', comments at all, they have such a large scope either way (to distance themselves from the original trilogy) that they can almost do whatever they want, posessing only a few constraints (the biggest problem about setting it afterwards is accomodating the three endings and avoiding a cannon but if they just set an ending in stone that's avoided, they will obviously need to reference these somehow if it's afterwards because it was a colossal event and cannnot simply be ignored).
