Figure I would quote both of you... see my response below.
Yet by having Clemont and Bonnie(whom you swap in for Brock) you run the risk of falling into the same trap that Sun and Moon has fallen into: Too many characters from which one or two get left behind in favor of Ash and whomever is the main "Female" lead that particular day. Ash, Serena, Clemont, Bonnie AND then Lillie? Seems to be one too many no and who would be the main female lead? Serena or Lillie?
If you're going to bring back old characters for a group, you have to have the quasi-leader or the mature one. For much of the Anime, that was Brock despite his Casanova mindset. Over time, and this was quite noticeable in Diamond and Pearl, Ash grew into a more capable trainer and frankly a Leader to which Brock was no longer needed(and the fact he had been on the series for 12 seasons and roughly 600 Episodes(that's a lot)). So while Ash filled in the role as the mature leader in XY, Brock also served the role as group chef/cook. That's where Clemont came in and essentially was his role outside of being one of the obstacles for Ash and Comedy relief(be it Science, Wife gag or Chespin). Same deal for Cilan in Black and White by the way, although Cilan was definitely the mature one with Ash regressing the way he did.
That's why if a character was to return in Generation 8 in any long term capacity, it would be Mallow in my book. She instantly fills the role of Chef for the group, can fend for herself if need be and is a natural leader(plus she can be an asset for Serena when it comes to taking things to the next level with Ash(Bonnie, despite being wise beyond her years, can't quite give the same perspective as someone who is much older like Mallow)). Then you add in Ash and Serena to make a traditional 3 character group with the unique aspect that two are Female.
Ash would be heading to a new region to challenge Gyms and the League(yet again). Serena would be joining him to pursue battling the regional Gyms like him(aka Female Lead going the same route as you would in the games(a refreshing change may I add). Plus adding in Amourshipping into the mix with a Female rival who takes a liking to Ash much like Miette and Macy) and Mallow would be traveling around the world exploring the various flavors, recipes and dishes the world has to offer. How else can you make your Family's Restaurant world renowned if you can't develop your cooking knowledge beyond just Alola recipes. The only reason I bring Mallow up is because of her Brother(he's doing that very same thing right now).
So to conclude, my ideal Generation 8 group(if its even possible) would be Ash, Serena and Mallow. You get the Amourshipping flavor that fans loved, Serena taking on a far bigger challenge than any Female Protagonist has undertaken to date and Mallow filling in the role as quasi-leader and group cook/chef. You could add a 4th but there would have to be a darn good reason for doing so.
