Is there a BIPs roadmap somewhere? Which BIPs are already implemented by bitcoin-core and which are scheduled to be supported by the client in the scheduled releases? 

I found these questions to be relevant:


Where can I find out which BIP (bitcoin improvement proposal) are going to be implemented or not?
What BIPs are supported by the standard client Bitcoin Core?


but still didn't manage to get an answer to that question.
All the BIPs that have been written up are listed on the BIPs Github project along with their status:

https://github.com/bitcoin/bips/blob/master/README.mediawiki

There's rarely a clear plan as to when they're going to incorporated into bitcoin-core and the corresponding schedule. In open source projects, features get incorporated when someone gets around implementing them and the review process is done.

The best way to follow implementation is to follow the development mailing list and Github pull requests (https://github.com/bitcoin/bitcoin/pulls).