With a party-based RPG, I don't really think theres all that much that needs to be changed to accomodate a second player. Your simply a member of the party, while the PC is still the PC in terms of NPC and quest interaction, your like the other NPCs in that your there for the ride and to help out (albeit alot more quiet). In terms of implementing the RakNet code in Unity3D for multiplayer, that would indeed take some dedicated development time, which is why im leaning more towards this being something experimented with in one of the expansions.
But atleast it would be worth the extra effort in my opinion, and once that functionality is in place, it should be relatively easy to implement in other expansions and the base game due to the nature of how coding in multiplayer support in RakNet/Unity works and would be well worth some extra cash to play with friends again in a great isometric party RPG.