I would love if some units could be combined with others, although I think it'd probably take a lot of code work on the dev side of things unfortunately, as I haven't heard anyone say its possible to mod it in 
For instance ... a single Dragon merges with a champion. The champion is now riding that dragon. Perhaps this dragon-riding champion now merges with a unit of spearmen. He is now that unit's "commander"
In case of Mount+Champion, movement speed would be determined by the mount while Initiative would be determined by the rider (or possibly an average of the two ... probably just the rider tho). Still though, Like riding any other mount, Riding a Dragon would probably reduce weight capacity/increase base initiative.
(either using the mount's movement or increasing the base move of the champ could work, but I think using the mount's movement might be a bit cooler)
If using a melee attack (or counter attack), you would get values for both the mount and the rider. If using an ability though you'd have to choose. For instance, either cast a spell OR use the dragon's breath.
HP would probably just be combined.
---> attaching a champion to a unit of spearmen however, may be a bit more difficult. Probably not necessary unless unit sizes got bigger (perhaps a lot bigger), but if attaching became possible then unit sizes COULD get bigger 
movement and initiative would go by the slowest value (probably that unit of spearmen) BUT!! a general/commander could give a bonus to its attached unit. For instance, a champion could have the trait "Forcemarch" which increases movement of his followers by 1. Also, presence of a commander could naturally increase the initiative by a couple points.
If a ranged commander was added to melee units, then he could still attack from a distance, but his followers couldn't participate in the ranged attack. (only his specific attack value would be used).
However, if a ranged commander was attached to a ranged unit (either magic or arrows) then any ranged attacks would be done together.
