We know both the trilogy and Andromeda will play a part (BioWare hints).
We also know BioWare will tell the story they want to tell, it's not going to be a 'fan requests compilation' (fans want different things anyway).
Liara was 106 at the time of ME1 (barely more than a teenager).
She could well have travelled to Andromeda (frozen) so even 600 years is no big deal and have come back the same way, so 1,200 years.
If BioWare wanted some 'old' crew they could simply have travelled along with.
After 1,200 years the events of the trilogy endings may have ended up approximately the same place.
Destroy - The AI lifeforms built up again (as the catalyst predicted) 'your children will create synthetics and then the chaos will come back'.
Control - Peace arrives but for how long will the Shep-Control entity last?
Synthesis - There is a new DNA, but evolution takes its own journey to who knows where.
I don't think it is a given that Shepard is back - we've only seen a bit of armor.
If that were to happen, being brought back from the dead or cloned has been done before.
So lots of room to press ahead in frankly any direction they want to.