Go for an HTTR expansion first since you've already done part of the work (besides I never bought HTTR myself

). I think it's a wise move to get another product on the Market (Garden

) fast.
ORIGINAL: Llyranor
... a standalone expansion would be beneficial to everyone
In my naïve but enlightened way I would suggest the following market scheme that would benefit both Panther and your customers:
Make the HTTR "expansion" a stand-alone game. Include the BFTB tutorial videos + 1 what's new video. Including the original tutorial videos they will have the additional value as an advertising for BFTB. Why not include the BFTB demo on the DVD if possible to give another taste of BFTB.
Set the price in parity with BFTB for people who don't already own BFTB but make it much (= MUCH) cheaper for those who already payed premium for BFTB.
Along the same lines sell BFTB at the same reduced "expansion" price for those who bought the upcoming re-vamped HTTR "expansion". To me this seems to be the fair way to market these games.
I guess you could go along much in the same lines with an updated COTA. Let newcomers make a heavier investment in the series but be kind to those who made that investment.
Regarding the sequencing of games I am confident I will enjoy whatever comes along in almost any order.
I second the idea I of making a Blitzkrieg-"expansion" containing both Poland, Belgium, France and Norway (Denmark was no real fight IIRC). Then again what about French doctrine? You won't be able to use the BFTB engine unmodified for French doctrine as little as for Soviet. By this I'm trying to say that it might be a bigger undertaking than "just" making maps, estabs and scenarios.
An eastern front major new game would be nice. You could probably get quite a number of expansions out of going east covering Finland to Caucasus (the old boardgame "Black Sea - Black Death springs to mind) to Budapest and Berlin. It feels like Stalingrad and Kursk has been done to death but that also goes for the Bulge so you might be able to pull them off with the superior engine and forward thinking of yours. Do a Firebrigade remake!
What about making two Mediterranean games:
1. COTA+North Africa (that would exclude an updated original COTA).
2. Italy: Sicily, Anzio etc. I'm not deeply versed in the Italian campaign history but since there were no real break-throughs (?) it might be a dull game except for the aforementioned battles.
Normandy? For sure. A bit one-sided but still interesting as a sim.
In the end I guess most of the grogs would like to be able to pick more or less any operation of WWII. Just keep 'em coming!