My suggestion....
1. You can make 2 series - one for old school fans who want to reminisce about the past and want to watch Misty, Brock and Ash get lost, and lost, and lost
2. Bring back characters - though there would be a big feud about who to bring back... Oak, Gary, Misty, Brock, May, Max, Drew, Harley, Dawn, Ritchie, Tracey etc....
3. Don't watch the show if you are going to complain that "So and so isn't back" etfc
4. They just have a show with JUST Ash to see how his training goes, and in the end, has a big tournament with all his rivals. This can bring back Butterfree, Primeape, Haunter and all his pokemon who he left or gave away.