There's a limitation to the DAG model that has bothered me for a long time.

I don't always want a branch to descend from a commit - sometimes I want it to descend from another (less featureful) branch.

I want the ability to say that branch 1.1 is equivalent to branch 1.0 + {some set of changes} is something that would be exceptionally useful in a lot of circumstances.

(And I know this would create some new fun and games from a conflicts perspective, but I think it'd be worth it for any use case that involves long term maintenance of different releases of the same piece of software.)