Those two things you mentioned are the only things that would work
The map desing and the unit transfer between maps.
The problem is the rest of it.
First the world map in war of the ring, which has completly different missions, and has a completly different system as in BfME1.
Secondly, everything concerning the maps (except the map design) can't be used anymore, because there are many new or different units, heroes and objects with quite different values.
So basically including the BfME 1 campaign would be the same as completly redoing it, and would still have some serious problems.