I am surprised how much I liked Heptarchy IV. It has great playability. Two reasons for that I think are the number and positioning of land territories. The sea territories are decent, but Sengoku Jidai has a really creative way of handling the sea terrirories that make it another really playable variant at this size, which I recommend referring to for ideas on how to structure sea territories: http://www.vdiplomacy.com/variants.php?variantID=27 . I would take one territory and work out how you think the nearby territories will make play interesting, and then work your way out to the rest of the map from there, e.g., consider London, the North Sea, Beligium, the English Channel, Picardy, Brest, the Mid-Atlantic, the Irish Sea, Wales and the powers with interest at the opening here, and how they work together and make the game really interesting; how they affect the balance of power in that area; how the navies and armies work together there...
I like balance, and tend not to like playing heavily imbalanced variants as much, but completely balanced variants tend to be boring. Some advantages and disadvantages for each power makes things more interesting (especially by mixing these up, e.g., one power is all fleets, one power could really use a second fleet but doesn't get it to start, one power has a chance to grab an extra neutral, one power has poor defense on one front... ), and a few -- perhaps stronger, or well-defensed or well-armed -- central powers can make things interesting for everyone.