Where can one find the Bitcoin core roadmap? Is there a site where it shows the current Bitcoin core roadmap on what features will be implemented in the future?
Refer to https://bitcoincore.org

It contains plans for releases, features (sometimes ) announced on the blog, IRC meeting notes and you can also subscribe to announcements here: https://bitcoincore.org/en/list/announcements/join/. Note that if you want to see which issues and PR's are scheduled for the next release, you can search by milestone. Eg. is:open is:pr milestone:0.17.0

Potentially the most up to date (and accurate) would be the Bitcoin Github Repository. Potential features and bugs can be found on the Issues page as well, and code that is proposed for merging on the Pull Requests page. It will usually also include comments from collaborators that can give an idea of when it might be implemented.