Adam Muto didn’t lay out Marceline and Princess Bubblegum’s fictional futures post-Adventure Time, but said fans can watch the kiss and “construct what their life will be like going forward.”
One could speculate Bubblegum and Marceline wouldn’t be around that far into the future anyway, although with Marceline being a vampire and Bubblegum being, well, bubblegum, who’s to say they aren’t still kicking around deep into the future?
Whether Muto is being cryptic about the future of the characters in case there’s any chance of a spinoff down the line is unknown, although nothing has indicated that beyond the introduction of Shermy and Beth.