If I were doing it I would gradually work on including everything (other then the Unseen) that existed from before the Clan Invasion.
It might look something like this.....
Base Mechs (the 34 standard Mechs from the 3025/3039 Technical Readout)
Open availability.
Light (7 Mechs)
Commando, Javelin, Spider, UrbanMech, Firestarter, Jenner, Panther
Medium (13 Mechs)
Assassin, Cicada, Clint, Hermes II, Vulcan, Whitworth, Blackjack, Vindicator, Centurion, Enforcer, Hunchback, Trebuchet, Dervish
Heavy (6 Mechs)
Dragon, Quickdraw, Catapult, JagerMech, Grasshopper, Orion
Assault Mechs (8 Mechs)
Awesome, Charger, Victor, Zeus, Stalker, Cyclops, Banshee, Atlas
New Mechs (the 9 new designs from the 3039/3050 Technical Readouts)
You have to do something special to unlock these.
Light (2 Mechs)
Raven, Wolfhound
Medium (2 Mech)
Hatchetman, Wolf Trap
Heavy (3 Mech)
Axman, Caesar, Cataphract
Assault (2 Mechs)
Hatamoto-Chi, Daboku/Mauler
Star League Mechs (57 rare Star League era Mechs from the 2750/3039, 3058, and 3075 Technical Readouts)
Extras purchased with RL cash. Rare designs to be gradually introduced post-launch.
Light (7+5+2 Mechs)
Phase 1: Thorn, Mongoose, Hermes, Hussar, Flea, Hornet, Falcon
Phase 2: Mercury, Firefly, Night Hawk, Spector, Talon
Phase 3: Jackrabbit, Firebee
Medium (4+3+4 Mechs)
Phase 1: Sentinel, Wyvern, Crab, Kintaro
Phase 2: Hoplite, Starslayer, Lynx
Phase 3: Kyudo, Gladiator, Toro, Phoenix
Heavy(6+4+8 Mechs)
Phase 1: Champion, Lancelot, Bombardier, Guillotine, Black Knight, Flashman
Phase 2: Exterminator, Cestus, Excalibur, Shootist
Phase 3: Galahad, Helepolis, Von Rohrs, Battle Axe, Hammerhands, Koschei, Icarus II, Crossbow,
Assault (4+6+4 Mechs)
Phase 1: Thug, Crockett, Highlander, King Crab
Phase 2: Spartan, Striker, Emperor, Nightstar, Pillager, Thunder Hawk
Phase 3: Atlas II, Mackie, Xanthos, Ymir