I kinda miss Ash-tachi and Team Rocket having an actual dynamic, instead of the boring good vs evil dynamic they have nowadays.
The episodes when they had to work together for something were always a treat. They always reminded you that TR were human (well, sans Meowth, of course), and showed that Ash and friends weren't always 100% squeaky clean nice people who'd never do anything questionable. I think my avatar says it all on that front.
I might be wrong, but the Beheeyem episode in BW was the last time they actually worked together, wasn't it? And I thought that was a really fun episode.
I did enjoy what they did with TR in Sun and Moon, but I did wish they had more episodes with Ash and friends.
There are others things I miss, too. OS took risks with some of its episodes. Like, if I listed all the weird crap that happened during Beauty and the Beach, you'd think I was describing an entirely different show. While I don't think Pokemon should return to that, the scenarios and jokes nowadays are very safe, and I think that limits the show a lot.