I think of the 'big 6' being such (rightly or wrongly) because of two reasons:
1) when one or many of them are on the startlist, they are the overwhelming favourites to win (and indeed do win in a high proportion of races, leaving scraps for others)
2) they can win on a variety of terrains, or types of races (stage races, one days, TTs, hilly, cobbles etc)
So when thinking of the 'next 10', I find myself thinking of riders who would be the favourites/most likely winners if the 'big 6' don't turn up, or are their closest challengers when they do, across a variety of the big races (ie not looking at winners in the Asia Tour races ).
The obvious candidates for me are Pedersen, Phillipsen & Milan - all are frequent winners, who can compete in sprints and classics.
Riders like Kooij & De Lie are similar, but I feel like they need more success in the big races to be included in the 'next 10'. Likewise Thibaut Nys who perhaps needs another successful season and races vs the big 6, to be included.
In stage races / climbing races, the equivalent obvious candidates from the past season or two are Adam Yates & Jorgensen. Riders like Carapaz & Alaphilippe are too inconsistent for me these days to be included.
There's then the riders who are versatile & often considered favourites or are very visible, but don't actually deliver many wins - Pidcock & Healy for example. Narvaez too.
And finally, there's the prolific winners but one-dimensional riders, ie sprinters & TTers - Tim Merlier & Ganna being the only real candidates over the past couple of seasons.
So I'm gonna go for the following list, being a mixture of the above. But I'm struggling to fill the last two spots. So let's call it my 'next 8':
Mads Pedersen
Jasper Phillipsen
Jonathan Milan
Adam Yates
Matteo Jorgensen
Tom Pidcock
Tim Merlier
Filippo Ganna
Well, you can easily add Ayuso (whom I rate higher than Jorgenson) and Almeida and complete the list.
Also, I'd put Pidcock out and include Hirschi. Similar profile but the Swiss wins more often.