After much more walking and much more dead they reach the Canadian border only to be confronted by a massive HUUUGGGGGEEEEEE wall.
Our heroes who’ve traveled a perilous journey to the north beg the guards on the wall to be a given a shot at a better life - as Rick likes to do he really spells things out in very clear ‘metaphors’ to show how they are now like mexican immigrants. The guards then open fire. The end.
Not as bleak ending/slightly far fetched/cheesy as hell so likely to happen in TV terms:
Rick’s daughter is some sort of miracle baby who due to being born after the outbreak is immune to zombie-ism. So is Maggie’s kid. Between this and a 28 days style thing of walker eventually starving to death the future is okay. Time jump to 15/20 years down the line and elderly Rick and Michonne sit on a front porchwatch their kids doing farm work on a sunny day.