I dunno what will be the ending. But if (assumptions) all Digimon has to go back to the Digital World, why?
If the producer company want to start over a brand new season without linking to this season, I don't agree to this reason.
A very good example is season 1. The kids left the Digital World, their Digimon partners stayed behind, the portal was sealed. The producer company has ended the season 1 with good ending.
We may think that season 2 will be a brand new season but we are wrong.
In season 2, someone itchy hands opened the portal, made havoc, new chosen kids were elected, new adventures began. Finally, all Digimon could stay on Earth and lived happily with their partners. My question is: why not make a brand new season 2 without relating to season 1? It is liked re-writing season 1 ending.
In season 3, all Digimon also returned to the Digital World but the portal was remained open in the last episode. I really no ideas why so. I dun mind all Digimon returned to their own world, but I felt not happy with the part where he found the portal in Guilmon's cave. Why not end at the part where everybody grows up, who married who and who did what jobs?
And now season 5, what is the real reason why Digimon cannot stay on Earth? To make a new season? But in new season, someone who is itchy hands may bring them back to Earth.
My point is:
Season 1 ending is perfect. Portal sealed. But season 2...someone somehow opened the portal, spoiled and re-wrote season 1 ending.
So, even in season 5 all Digimon returned to their own world, the producer company MAY find a reson to bring them back in season 6.