What VCS?
This isn't a "What VCS?" thread. Its a "What should our workflow be going forward?" followed by "What tools support that best?"
However, since the question will come up:
Agile feature branching is necessary to support a feature-branch driven workflow.
This probably rules out svn for this purpose (when was the last time you heard agile or fast used in the context of svn?), leaving p4 or a dcvs like git/hg.
We have been doing feature branch based development on the side or quite some time. The entire perforce.freebsd.org repository exists because of this. If you look at things like the AMD64 port, lots of the SMPng, network stack renovation etc.. those lived as p4 feature branches before being merged into head as a functioning, going concerns.
However, I don't think p4 is an option given their second rate command line interface and unwillingness to fix 10 year old regressions or make basic quality of life improvements, not to mention being closed source and the scalability of keeping client state on the server.
As for dvcs's it seems fairly clear that the community at large is falling behind git. Yes, there are a few high profile exceptions, but in general the convergence seems to be on git.
Git wasn't designed with our freebsd.org-1.0 workflow in mind. However, the question is whether freebsd.org-2.0 and git can put us in a better position. I think it does.
Assuming that we tear up the rulebook for "the way things are done" in freebsd.org universe, we could have a git repo as the root repository to hold head, and everything else is downstream from that, including release engineering. Use the best available tools to support each job at hand.
The big downside to Git is that it is quite alien to the way we've traditionally done things. There is a very high culture shock value.