In fact COA will include a very big new feature and HTTR didn't respect to RDOA
EDIT:
COA won't be only an "scenario pack" including new scenrios for the Market-Garden operation...
As said it will include at least one MAJOR feature, with many more tweaks and additions apart. And a totally new game based on a diferent time period and different campaing, with different units and weapons fought over different ground (if the making of Greece, Creta and Malta is not enough new ground, what do you want the full eastern front?)
Sincerly, don't want to sound fanboyish but I don't know what more do you want, as it is the policy many times stated by Panther Games:
-One major feature per game (Campaing Mode if done correctly can be a very good and popular feature for the inmersion factor).
-Based on a new battle/operation in a different ground.
-Many tweaks and additions.
If you check RDOA and then play HTTR you will note the diferences between both in an huge amount of smnall changes, which makes playing both games, if not totally different, at least a different experience and fighting. Wargames are not FPS, where primary the only changes between each gme is the graphic engine and only with that you make a lot of difference, adding and changing radically a game system within wargames is not possible. Just check ANY wargame series (you yourself said HPS), the changes go slow at strategical and operational wargames.
Regards.