I would have to say...every 3025-era design that won't cause legal issues. The more the better. In particular right now lights seem to be a bit underrepresented.
Ideally I would like to see a minimum of 2 mech chassis for every tonnage possible. That way you can't immediately tell what something is the instant your sensors pick it up, just by tonnage. IE, with the mechs in the game now, if you see a 20 tonner pop up you know it's a Locust of some sort. If an 80 ton mech pops up, it could be either a Victor or an Awesome, which need to be handled pretty differently. You need to figure out what you're dealing with pretty quick in that case. It adds a bit to the game to need to take your recon very seriously. In the case of the 20 ton mech it would be cool if it could be a Locust, or a Flea, or a Mercury (yes there is a non-Lostech variant), or...whatever else.
A couple specific designs that tend to be forgotten about which I wouldn't mind eventually seeing though:
Whitworth - a small cheap missile boat
Clint - it has the same movement profile as a certain (unmentionable) very nimble 45 ton recon mech. The only real advantage the other guy has over the Clint is that it's a lot cooler looking, really. The Clint could be tweaked to fill the unnameable one's role just fine, and with the rejiggering of autocannons its default configuration would probably be pretty viable.
[not speaking to the rarity in each location, just in production at 1 or more facilities throughout the IS.]
