I have to put in a vote for some British vehicles - (playing Brits & Germans means I don't really need Americans or Soviets) - Pinky's request for a Valentine is a good one, there's plenty of uses for the chassis, with the Bishop (ugly as it is) & Archer being obvious choices. That it covers the whole war from 2pdr/3" howitzer armed MkI's to the 75mm toting MkXI, so any Brit (or Soviet) player could include one.
The Western Desert is quite a popular gaming arena so the various Cruiser models would have a place on many shelves, along with Matilda II's.
Personally, with my focus on late war & post 45 I'd really like a plastic Comet - given late war's popularity I'd imagine it'd be as popular as a Cromwell.
(I've just picked up WG's new resin Comet & it's a bit of a curates egg - the casting is nice & clean, it goes together nicely, but it appears to actually be a Comet B - a post war version - fish-tail exhausts, no port on the turret roof for the smoke launcher - but no banks of smoke launchers for the B's turret sides either & no lifting rings on the upper edges of the glacis.)
Cromwell & Churchill - well they're undeniably popular choices, so surely there's room for more than one in the market?
Challenger/Avenger - despite their sharing a chassis, this one is a big ask, as they'd have to be a stand alone kit, not a Cromwell add-on, but are they less desirable to gamers than the M10/Achilles? (I realise only 200 Challengers were built & nobody seems to know how many Avengers - probably fewer than 100 - but fewer than 500 Tiger II's were built & no-one questions their suitability as a subject.)
For the Germans, the SPG & Jagdpanzer variants of the PzIV would be great & I second the desire for flak variants of assorted halftracks.
I'd welcome any or all of these as Rubicon have demonostrated their ability to produce a superior product.
(How about this for a double kit
)