Canon doesn't need to be set in stone but sounds like doing so might be easier/preferred to you, which is fine in my book. I think our easier route would be to play Grey Wardens who did survive but I do think recruiting our own civilian folk would be fun, as you said. You mentioned playing grunts which I think may me wondered about the ones who got to run off when the PC let them go as opposed to killing them. A number of different people could join the cause and it's entirely likely we could follow the Party's movements but always be a few steps behind so we can see the aftermath of main events. It would also give us room to do our own thing, pick up our own side quests of sorts and do our own to clear the Grey Warden name, so to speak.