After the 5th season there is no more content, it only remains to read GPX; I know that the IF Doujins would be the continuation but there is really nothing else and I have seen info about the son of wahsu among other things.
You're pretty much caught up, then.
Paradise Wars prequel. Its been announced. Kajishima-sensei also talked about it in his latest doujin. He also called it the sixth season. So who knows.