Similar sets as 151 and Black Bolt & White Flare, but for all other generations (2-4 and 6+) - especially gen 3 (and 2), since I’d love to see a FA Seviper. 
Also, to quote a different QotD from a few days ago:
A full set of just Ash’s Pokémon would be cool to see. And they could include the pre-evolved forms of Pokémon he evolved himself, instead of just the final teams of each season pictured above (except for Primeape 
).
Greetz,
Quuador