So the challenge, to me, is that if you fork docker, then people are going to have to get used to a different command structure anyway as the forks will diverge.

One of the reasons to avoid a fork is the confusion caused by two kind of similar but not quite the same implementations that diverge slowly over time.

rkt is unlikely to just copy the feature set of Docker, but they provide competition in implementing new ideas and as it's not intended to be identical there's less confusion.