idea is very interesting, it would indeed bring COH2 interest much deeper allowing for more choices so more flexibility in the commander (and I dont think it would worsen the commanders balancewise) but I understand correctly ur suggesting to merge the commanders. If so, I think it might complicated to do so from since those were paid DLC
Id like to see you go further in the idea, eventho it might stay just theory, giving more examples as you did in the OP, to see with how many commanders you will end up with and which commanderes abilities you would find intersting to mix to create brand new commanders.
I think there are enough abilities for 6 commanders for each faction(with a couple of stragglers), if the abilities were just taken and rearranged.
Its highly unlikely though, that 6 balanced commanders could be made that way. The suggestion is much more of an overhaul than that. And given the DLC model, it would also require the devs to flood enough unique abilities into the game to create new commanders so that they could offer up replacement dlc to people who bought the other commanders. Otherwise, there's just going to be hell of a lot of bad feelings unless relic waits a good year before it does something like this.
While I would have liked to see the designers apply no overlap from the start, and would also love to see ability trees(I particularly like whiteflash's partial tree suggestion) I don't know that I agree with its implementation at this point, UNLESS relic adds more upgrade content to each faction, which they definitely should do. Some abilities could definitely be added in a non-doctrinal way, like as Tommy suggested, ostheer vehicle smoke as a global fuel upgrade.
Unless relic adds this sort of content though, the ways to play are going to be greatly reduced, because currently its in the commanders how a player can tailor his style and unit composition, opening up or closing off certain builds, depending on what the doctrine choice offers.