There is a big difference between too insignificant to be covered and completely unreproducible with rules and magic in terms of weight with abilities beyond reasonable disbelief.
My point is that something like a Jeep probably isn't significant enough as a vehicle for it to matter. I've played a bunch of different game systems and so I'm okay with using different mechanics to represent the same thing.
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If I were to rewrite it, I'd start with 3025 technology and balance it as best as I could. Start there, and include infantry, tanks, aerospace, and balance all of it. I'd probably use a combination of BMR and TW for infantry and vehicle rules. I'd combine the bonuses (but not the penalties) from a lot of the specialty ammo (tracer, flak, and flechette) and give it to ACs for free. So -2 to hit against VTOLs and fighters, +1 for night fight instead of +2, and double damage against infantry. That would all be part of "standard" AC ammo and would explain why so many mechs had them. Suddenly they become really good weapons, versatile even if they are a bit inefficient against mechs. Double-tapping rules would be standard.
I'd have an infantry scale game similar to Battletroops (I'm probably the only person who liked that game), but I'd make it similar to other 28mm games like Infinity. There would be a clear connection between the rules in the troops-scale game and the rules in the mech-scale game.
I like my Robotech mechs, where they are mobile and they can do drop kicks and throw girders at each other, so I'd give rules that favor those over simply the walking tanks of the computer games. There would be a physical attack for grabbing a vehicle and flipping it over -- or if it is light enough, grabbing one and throwing it. Cinematic play would be given a preference over realistic style.
I'd rework the top of the engine chart as well, so that a 400 rating engine doesn't screw you over as much. Maybe the 400 would weigh in at 38 tons or something instead of 52.5, and scale down from there.
I'd base infantry around the squad. So an SRM squad is 7 men, has a leader, two SRM guys, and four riflemen. Each guy adds something to the squad, even if it's just a body to take damage. While a medium laser might kill a random guy, something like a machine gun might simply just kill a squad outright (Damage: one squad). So if you get lucky, your 2 medium lasers might kill the two guys holding the SRMs, and then the squad can't do anything to you. Or you might just kill two riflemen and the squad is pretty much unharmed. You could have a platoon of multiple squads, and that would increase their firepower, but then anti-infantry weapons (infernos, autocannons, machine guns) would have a chance to hit multiple squads at once.
I'd dramatically expand the quirks system, and give some of them significant in game effects. Each mech and vehicle would have its own standard quirks. Some might be good (the Rifleman's anti-air capabilities), and some would be bad (say a vehicle that can't fire when moving at more than cruising speed). I'd be liberal with them too, to try and make the rules match the fluff. Some types of "advanced technology" (like rocket launchers, a-pods, maybe anti-missile, early active probes and ECM, alternate ammo and missiles) would become standard technology. Let's give the 3025 players a lot of good options.
Anyway, I'd get 3025 technology balanced and working exactly how I wanted it. Then I'd move on to advanced equipment. A lot of the advantages of Star League tech would be handled with "good" quirks. "Improved heat sinks" might give you +5 or +10 heat dissipation every round, instead of double heat sinks. "Extralight engine" might give you an extra Walking MP. "Ultra AC" might just eliminate the jamming from double tapping. "ER Lasers" could add 1 to all ranges. And then certain types of advanced technology would have their own game stats. Gauss Rifles would obviously need their own stats. Ferro-fibrous armor might just be 18 points per ton. LB-X ACs would probably be lighter with longer range. I'd probably give them a -1 to hit but do the cluster rolls in 5 point locations.
The goal would be to give SL mechs some pure advantages over baseline Inner Sphere stuff, and give a justification as to why certain mechs (like the Marauder) seem to be designed badly -- they originally had SL quirks that made them very effective. When (and if) Clan stuff came out, they would continue that trend. They'd probably get lighter and better weapons, but it would be the mech quirks that really set them apart. The goal would be to give the Clans a big advantage, but not insurmountable. Also BT really needs to trim down the weapon charts. They are insane right now. Pulse lasers also need to be nerfed, especially Clan, because it's 25 years later and they're still the best weapons in the game.
As far as including all these quirks in the construction rules, I might say that the average IS mech has, say, 2 pts of positive quirks. The average SL mech has like 7 pts, and the average Clan mech has like 10 pts.
