Comet was essentially a revised Cromwell, so it should be possible to dual purpose a Cromwell kit to make Comet as well. Some Cromwells did have the revised type D hull with the Comet style hatches and 15inch tracks, so that's not a problem. So you need an extra turret, extra gun, and separate upper track runs to include the comet return rollers. Given that Cromwell/Comet is quite a bit smaller than Sherman, I would imagine there would be room on the sprue.
From a commercial PoV, dual kitting would cut down on tooling, and widen the appeal of the kit.
Cromwell single kit = Uk 7th armoured, 1st polish, and armoured recce units of UK Armoured divisions
Comet = Guards armoured post Battle of the Bulge
Dual kit = All of the above
Edit: Actually, include the parts for the 95mm Howitzer as well, and you'll also get some of the Royal Marine players.
So that's two turrets, three guns, 1 mantlet and a pair of track runs. should fit fine I would think.
Of course ideally I'd like to see that most widely exported of british tanks, Centurion, but seeing as we are focusing on WW2 vehicles, I'll accept that I won't see one in any scale other than 1/35 for a long, long time.