First i'll start off by saying that the conclusion of this theory is definitely not gonna happen, second i won't waste your time and get right to the point:
At the end of All Hail King Julien the lemurs and fossa are allies
In the movie there is ~7 minutes between Alex the lion stranding on Madagascar and the fossa attacking the lemurs
7 minutes movie time can easily be worth 11 minutes episode time if you know how to write
In Merry Madagascar it is confirmed(?) that there's 300+ days between Madagascar and Merry Madagascar
300+ days can easily be worth 3 seasons (maybe even more)
So in conclusion: All Hail King Julien season 6 could easily exist
Of course it's not necessarily gonna happen, but this has been on my mind for months now so i just had to share it somewhere and this seemed like the perfect place to do so