There isn't really a roadmap, it's "finished" barring staying current and avoiding "bit rot".
In general Capistrano (v3, since 2013) is designed to be a stable core, around which a plugin ecosystem can exist without direct intervention or blessing from us can flourish. V2 was not very extensible, and did not have a bright future ahead, v3 as a ground-up rewrite warranted bumping the major version number for the first time in 6yrs.
Keyword stable; deploy-time infrastructure tools aren't the place to chase the hottest trends, many large businesses deploy money and downtime sensitive applications frequently with Capistrano and have come to expect few, if any surprises. As a rule we air on the side of extreme conservatism.
I am one of the three maintainers, and the longest serving (8yrs), but I believe I speak for us all.
I'm glad you're excited about Capistrano and getting value out of it, hopefully it will soon be boring, and you'll get 10years reliable service from it!