Never say never, but very highly extremely unlikely we will ever see Android 6 on FP1. Not even Android 5.
As you might know there is an ongoing community effort to have [Android 4.4.4 on FP1][1] and we follow and support that with great enthusiasm. Hopefully we can officially support that one day.
Security updates and fixes will continue to be released, but also depend on the severity of the bug and company resources.
Hope this helps you!
[1]: [UNOFFICIAL] Stock ROM Android 4.4.2/4.4.4 Kitkat for FP1 & FP1U