I've been sitting here thinking how I'd retool things to have more modern sensibilities, instead of being influenced by 80s sci-fi and anime. The core question is . . . why do they use battlemechs in the future?
I know "because giant robots are cool" is a fine reason to have giant robots in a setting. But since BT leans more toward realism than, say, anime mecha, I wanted an excuse for why militaries would field ambulatory robots with vulnerable joints rather than using tanks and planes and guided missiles, and why they'd need pilots instead of letting everything be controlled remotely. Any weapon or armor you could equip to a mech would normally be just as effective on a less-complicated vehicle.
Moreover, why is there so little use of computers in the BT universe? How did they go a thousand years without hitting the Singularity?
My excuse is a big change that ripples through the whole setting, but I think opens up some fun game mechanics.
Kearny-Fuchida Force Shields
Let's start with existing BT canon and tweak a bit as we go. BT has the Kearny-Fuchida drive being first tested in 2107, managing to fold space to transit a ship from Sol's zenith point to its nadir point. These jump engines have a core of titanium-germanium alloy, and produce a field that envelops the object to be transported. Dropships attached to jumpships have to be cloaked in a 'K-F Boom' to include them in the field.
In canon, mechs got invented in 2439. Obviously we will probably be able to field walking robots on Earth far before then, but with real-world tech they're not actually superior to standard vehicles. So in this version of BT, what makes mechs suddenly viable is the ability to produce lower-strength K-F force shields. This isn't enough to pull an object into hyperspace, but it is enough to disperse incoming kinetic force.
The main limit to its use is that you have to make a complete shell; you can't just have a hemisphere shield. If you try to use a KF Shield on a tank, you have to cover the treads, which means you don't have any traction with the ground. If you use a KF Shield on a plane, no air can enter the shield, so wings can't provide lift. The only form of mobility that does work is walking. (Pure thrust works too, so you can have aerospace fighters and drop ships with shields, plus mech jump jets.)
Tactical Consequences
Because KF Shields essential create a zone of vacuum around a mech, this explains why mechs are so bad at shedding heat. The electromagnetic forces of the shields would disrupt radio and light communication, so while you would be able to receive low-fidelity broadcasts for communication, you couldn't send enough binary data reliably to remotely control a mech, or to have guided missiles. It's not even safe to have active electronics on missiles you launch, because they'll be shut down by EM overload as they fly through your shield.
You could program your LRMs with a target, set them to standby mode, launch them, and have them turn back on after they fly a short distance (which explains why they have a minimum range).
KF Shields can absorb a lot of kinetic impact, but eventually they get overloaded. Single-impact strikes (like rail guns) and broadly dispersed blasts (like big explosions) are easier for a shield to absorb than a scattershot series of lighter strikes. Each strike draws power, and it takes a moment for different shield emitters to increase or decrease charge, so if you hit several times in a row, the shield can lose fidelity and temporarily collapse. This makes volleys of missiles and multi-shot autocannons superior to large cruise missiles and rail guns.
Lasers simply bypass a KF Shield and strike the target directly, but they produce a lot of heat, which means you only get a few shots before you have to withdraw to safety and shut off your KF Shield to cycle air. A light mech that can hit and run will want lasers. A larger mech might carry a few to try to score lucky shots, or for the utility of easily setting terrain on fire.
Finally, you have the 'big guns': gauss rifles, PPCs, and thunderbolt missiles. These weapons are usually conserved until a mech's shield is down. If the shield is up, a blow from one of these is only as effective as a single missile strike. But if the shield is down, they can cripple the target.
Thus, a common tactic is to have medium mechs pepper a target with SRMs, LRMs, and autocannons to bring down its shield, and then a heavy or assault mech delivers the killing blow. Mechs with lasers are useful for harrying targets, and occasionally they'll score a lucky crit.
Mechanics
This would of course shake up the game's mechanics. If a specific body part's shield takes enough damage, the shield shuts down there, exposing the structure (which here is anything physical - armor, endoskeleton, weapons, etc.). Since the rest of the mech's shield is still active and still trying to form a complete shell, the structure would be slowly damaged turn after turn from warping forces. If you're able to survive a battle, though, you can shut off the mech's shield to stop this damage. Then after a few minutes the shields that were depleted will recharge. I like this because it makes it more likely to continue a campaign with damaged mechs, without them starting a conflict completely boned.
Since your mech is shrouded in vacuum, it won't ever shed heat, so the heat scale only goes up. I'd probably have it so when a mech overheats, instead of shutting down the engine, it shuts down the shield. If you override, the pilot can get injured and ammo can explode. When you shut down the shield, your mech loses heat quickly as cooler air circulates in.
Different weapons would respond to shields differently. There'd have to be a lot of tinkering to balance, like, the ability of lasers to bypass armor altogether, but to be more likely to overheat you.
Now, since we'd be redesigning the game, I might also look for a more modern design sensibility regarding dice. Original BT has a lot of dice rolling, and no ability to aim. However, Mechwarrior video games reward aiming, as you try to take out specific parts of your opponents' mechs. I could envision some sort of pilot mechanic where your piloting and gunnery scores give you 1 to 3 points per round that you can spend to aim a shot or do some clever maneuver.
Also, maybe we'd port in something from Rogue Trader, where your attack roll was also your hit-location roll. (You'd roll d%, and then you'd look at the 1s digit of your roll, with 1 being right leg, 2 left leg, etc.) That could speed up play.
BTW, Why No Singularity?
This one is easy. We're just going to by fiat decide that the energy requirements of Artificial General Intelligence are so high that they're ultimately less efficient than human cognition. Physical limits were hit somewhere in the 2040s with how fast a processor can get and how many parallel processes it can do at once. To produce the cognitive ability of the human brain, you need a device that requires far more expensive energy than a human's food needs, and produces too much heat. Computers are still great at doing math and other tedious tasks, but they never become able to have consciousness, or control a mech with any great versatility.
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Okay, that's me rambling. It's kind of useless without a proof of concept.
