Sounds like sage advice. I would add that one could start with models that could be more or less easily converted from existing ranges just by changing a few bits (like the Elven Trade Guard from Militia).
If one of the moulding artists reads this, I might suggest something:
The most characteristic and defining parts of miniatures – heads, shields, shoulder pads and weapons – are at the same time those which appear on nearly all miniatures of the same range. If someone could mould a sprue containing, say, a Firstborn Knight´s helmet, round shield, straightsword and shoulder pads (and maybe add a crenellated axe blade for good measure), players could use this on models from other ranges, with a wee bit of sawing, gluing and greenstuffing, to create a wide range of Firstborn models: from City Militia through Light Horse to Mounted Repulsar Knights. Same with Elves, Devout or Blackbloods. Note how all Elves have the same style hood and gorget, and most Blackbloods carry spiked round Saracen-style shields?
I might also open a new thread showcasing those house-rule units I´ve already done stats on – if it´s OK with you.