I thought those battles were okay. I liked the strategies Diantha used in her battle and Iris vs Cynthia had some great emotional moments in it. But they mainly suffer from the same problems as the other battles; overly stiff animation and littered with one-shots. Not to mention Lance vs Diantha in particular had a weird lack of dodging, your basically just watching pokemon tank hits for 22 minutes straight and that's just not exciting to me.Iris vs Cynthia and Lance vs Diantha were both perfectly great battles IMO.
While the XY series has not mentioned any past series whatsoever (except for a brief mention of Dawn's Piplup in XY022), it would be very fitting to see past companions watching the finals of the Kalos league live. This isn't just big for Ash, it's big for the anime itself; a milestone that took 20 years to reach was finally reached. I'm sure some or all of his companions watch the leagues off-screen, as Max said he watched the Johto conference and Misty was watching the Ever Grande conference. Having his past companions cameo and witness Ash winning the league would be great, as Ash has been working on winning a league for a long time now and it would come full circle for all of his friends. I've actually been thinking for a while that they should show past companions watching Ash in the finals too; it would be so satisfying to see as a long time watcher of the anime.
A party would be cool too, but that would be a lot of characters to juggle at once. Plus, the Kalos group would probably have to go to Kanto with him, then leave to Kalos afterwards. It would be awesome though, as they can celebrate both the Kalos league champion and the one who foiled Team Flare.