I imagine the game running in a similar sort of way to Battletech, but on a grander scale and with more of a Powell and Pressburger-y aesthetic. It's also got a bit of FTL's crew shuffling mechanics, as well as the upgrades, points cost, and cornucopia of upgrades and bonuses for even the most teeny-tiny robot and its most junior commander taken wholesale from every mad roguelike I've ever played. Rolls are made using D20s, and there's plenty of them; I intend it to be like a kind of insane mashup of Battletech, Path Of Exile, FTL, Warhammer 40000, and Risk. Even if the last one is mostly because it'll go on forever. =]
The Thompson Drive is essentially a primitive fusion reactor, the initial incarnation of which was hammered together out of Weird Science, a lot of oddly-shaped metal, and the contents of Dr. Archibald Thompson (4th Viscount Kincaid)'s garden shed. The plans were recovered and refined by the British War Office in 1917 at the behest of Lady Thompson, who wanted it to be a memorial to her husband. It was supposed to end war, you see, in much the same way that dynamite was.
The fact of it is, occupational health and safety was about as much a concern for WWI-era military institutions as whether or not the spider taking up residence in the corner of one's bedroom answers to the name of Mildred. The British Army in particular was notorious for this, treating the common soldier (the extremely common soldier) as what amounted to a not terribly bright attack dog with a penchant for hard booze and women of easy virtue. Thus, in order to squeeze as much power out of a mech's Thompson Drive as possible, nimby-pimby things like radiation shielding and guard rails are left entirely by the wayside in favour of more armour, more guns, and more ammunition. However, individual crews spend a lot of time in their mechs, and tend to cobble together their own versions so that the "stock model", as it were, is rendered safer and more user-friendly. The uncharitable might compare the modifications' collective utility to putting a leash made of used dental floss around the neck of a maternally outraged grizzly bear, but it's better than nothing. Probably.
Essentially, every faction has access to everything from 1-inch naval guns to War of the Worlds style heat beams, but not every faction has access to heat beams that actually, y'know, work. At least not all the time. Building a mech is about tradeoffs. If you want heroic and gallant officers who do their duty with the courage and honour of a particularly fervent Ultramarine Chapter Master, you have to forgo the laser zap bolts. Conversely, your superheavy stompmonster might have all the bells and whistles you can cram into it, as well as a few more you nailed to the outside, but it would end up being led by some unspeakably inoffensive stuffed-shirt with delusions of adequacy. Or it could have both of those positive attributes, but run the risk of turning into an Extremely Not Cheap crater every time the chief engineer so much as looks at it funny. It's really up to what the individual player wants to do.
See, to my mind, you can get roleplaying by proxy out of a crunch-heavy game just by giving the player options out the wazoo. Two players from the same faction might build completely separate armies - one might theme their force around a grizzled, wily old mech captain hurling his ancient war machines into one last battle with every trick in the book at his disposal, and the other might prefer a small cadre of highly experimental mechs forced to enter battle as a last resort and captained by people whose field experience chiefly involves hiding under something heavy-looking and waiting for the horrid bangy noises to stop. A third might do something totally different. It's really up to the players! At least, that's how I envision it being.
As far as weapons go, they're a mix of actual artillery, converted naval weapons, big rockets, scaled-up infantry equipment, experimental weapons that would otherwise be impractical, and a boatload of Weird Sci-Fi Weapons with a coat of WW1 paint slapped over them (which is presumably either khaki, mud brown, or very, very red). For example, Thompson Drives don't lend themselves at all well to miniaturization, so they're no good for aircraft, but the biggest mechs can also function as aircraft carriers that fuel battery-powered biplanes and monoplanes with the energy output of the Drive. Those planes might only have an air-time of 15 minutes, but that's more than enough time to act as bombers, spotters, and interference runners in the field. All the dashing fighter aces? They're mech captains now, or working for them as grunts piloting Lillehammer-style bomb-armed helicopters.
The point of that weird little story is that thanks to the Thompson Drive, however tangentially, a lot of the mad ideas of early 20th century science-fiction writers sort of work. The home front looks Edwardian, but the streets are lit with electric lights, the trains are all electrified, and in many ways it has the infrastructure of a modern nation (without the internet, obviously). We've even been to the Moon - though it was the British Empire that did it first, and not without blowing up a few dozen would-be astronauts. Grand zeppelins cruise the sky-lines of the world, calling at "air-ports" to recharge their batteries and let the great and the good take in the air of foreign climes. The Russian Revolution still happened... ish. The world is changing and the world is weird.
There isn't an iconic weapon yet, but in my head I have some quietly excited gentleman scientist showing the PM his plans for a new bunker-busting supergun that fires a burst of energy analogous to that of a solar flare. The squat, dumpy, orange-glowing thing is hooked up to a Thompson Drive, hums ominously, and after a few seconds of charging incinerates a stand of small oak trees roughly half a mile away. Thus, the Sun Cannon begins to be equipped to British mechs, and launches searing stellar plasma at the rampaging Boche. One imagines there's a picture of such a thing happening in the rulebook somewhere. =]
I hope this was of some use to you. If nothing else, it certainly was to me. =]