No one knows when or where it will be set. BW have been pretty tight lipped about it all, which is kinda annoying. There have only been a few vague comments that could be taken different ways.
It seems like most people - including me - are hoping for a sequel, which I think would sell better than a prequel or sidequel. How they would manage a sequel has been a topic of (too) much debate here.
Well, I don't like to add to the debate, but, IMO if they made a sequel (which I'd really like) they'd need to fix it 4 variables:
1. Whether Synthetics are still around
2. Whether Shepard's still alive
3. Whether the reapers have been destroyed or are hanging around in a benign fashion
4. Whether organics/synthetics have been combined (synthesis)
Seriously, can't these variables be handled by just changing a few NPC dialogue lines? In other words, change what some people say to acknowledge the choices at the end of ME3 but create a plot where the 4 variables aren't that important, also they could just 'fix' any changes:
1. Synthetics could just be rebuilt/the star child lied/the star child wasn't aware of how the crucible's targeting had been improved since the last cycle. In short, there are many ways that synthetics could be 'brought back to life'
2. Shepard can wither appear in a short cameo, or just be mentioned by an NPC
3. NPCs can mention them, and I suppose there could be a scene where the 'nice reapers' (synthesis) or 'controlled by Shepard's consciousness reapers' (control) help you against the new bad guys...
4. Isn't this essentially just a 'green glow' that a) could 'wear off' (the glow, not the synthesis itself, obviously) and doesn't really affect much? I mean, organics and synthetics aren't going to kill each other if their DNA has been combined, but surely they won't do it anyway, knowing how synthetics like the Geth and EDI aren't evil?
Quite frankly, I would have loved an ending where Shepard convinces the starchild/catalyst to only destroy the reapers and nothing else by explaining to it that based on what he's learned, synthetics and organics can 'get along'.
A Mass Effect 4 where you can meet Shepard or some of his crew in short cameos and visit a Rannoch hub-world where you see the Geth and Quarians co-operating would be great. So, ME4 should give closure to ME3's endings rather than just ignore ignore/side-step them. (sorry for the essay)