That said... I personally would have had an early setting pre-3000 as the initial testing bed to build the 'old' mechs up and ensure that they heavily populate the game, gradually implementing the newer models of later years after getting the initial balancing done in such a way that standard heatsinks are 100% viable as well as a number of weapon variants to suit both low frequency burst damage, high frequency DPS style, and middle-ground players. Eventually developing a repair and rearm model and a basic community warfare to a stable point, then shifting the timeline forward to 3030 to 3035 where the Helm Memory Core's influence would first start to spread, trickling the technology in gradually as part of CW.
A large focus for the IS would be role warfare -- not as PGI developed it but as I see it. Scouting, radar systems, stealth, fire support, concrete objectives (that mean far more than the alternative of kill everyone), non-mech vehicular and infantry A.I. (which someone using a Command Console can direct so long as both the pilot and commander are conscious/alive). Staying far more true to the lore, battlemechs will be very unique in not just what they can carry
but also in how they play. Some mechs have turrets. Some don't have torso twist. Some have arms that flip. Some have rear firing weapons (the Atlas S for example in Battletech has 2 rear mounted Streak launchers). Some have rotating cockpits (heads) that can look left and right, up and down beyond the movement ranges of the torso and arms that can aim farther out than 90 degrees from the front. (The King Crab in particularly is a mech without torso twist, but the visibility of the cockpit and the flexibility of the arms allows it to fire directly left and directly right, effectively firing in 2 completely opposite directions at the same time.)
As I mentioned, not only would additional armor make your mech larger (if you take a Victor and give it the armor of an Awesome you can expect to be quite 'expanded' like an Awesome), but you would only manage so much internal space for things like ammunition. So take an Atlas whose body could not even fit a proper LRM-20 but instead had a rapid fire, rapid reload 5-tube launcher, and you want to stuff on more ammo than it is designed to carry? They will appear as if mounted on the outside. For example a drum of ammunition for the autocannon. This of course puts a little extra value on the CTF 4X as opposed to the CTF 3D, as the 4X is described as having additional internal ammunition bins which soon became popular with the Davions over their own somewhat superior but less armored 3D design.
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Alongside that development, I would also have the Clans at Strana Mechty, developing their tech while they fight within the Pentagon (the five worlds) for their own exclusive balancing runs. Development of their own independent economy that would favor things like honorable combat, fighting while outnumbered, various roles in tune with the specific Clan. (For example the Crusader Clans would possibly favor 'bad guy' postures and behaviors such as offensive valor, aggressive combat, attempts to directly kill the enemy pilots, things of that nature. The Warden Clans would favor more 'good guy' postures such as effectively neutralizing enemy resistance
with honorable means and minimal casualties (to both sides; this means defeating enemies without resorting to the Crusader Clan preferences of headshots or deliberately legging enemies) and other activities not yet conceptualized by me.
Resources in the Clans are allocated with what are effectively permissions and/or trials. Rank is earned by defeating someone of a higher rank. Resource allotments are earned by how you perform in battle in relation to the Clan's expectations. You fight and do well, you get what you need. You run like a coward and hopefully before that moment you did well enough to cover your repairs and rearm after such dishonor.
Meanwhile the IS economy would promote teamwork, cohesion, lance-based play and so on. The factions in general provide you with both pay and some financial assistance in maintaining your mech (at least as far as what is kept standard in their eyes, there's no reason for a military to pay for your personal customizations). Mercs admittedly may find somewhat different mentalities based on how each merc unit decides to handle their personal economies. Lone Wolves coming to the aid of others in offense or defense calls are bound to be assisted within reason financially in their repairs and in compensation. Though compared to the lore some minor adjustments would need to be made so that newcomers are not overly pressured into joining units or factions if they don't wish to.
The Clan mechs would control very differently (most Clan mechs do not have functioning heads. This is a huge difference in favor of the IS, who can look and aim without exposing the entirety of their bodies). Clan mechs can still aim to the far sides with the mechs capable of doing so but this is without the rotation of the cockpit and instead with the twisting of the pilot's head. This can both be beneficial and detrimental on either side. One thing to note however is that most Clan mechs cannot use their arms to aim left and right due to the removal of lower arm actuators. Even so circling behind them is a risky move as they can flip their arms as well.
Omnimechs would have an entirely different R&R system over the Battlemechs. Loadout changes would be easy. Clan mechs in general would have a more limited assortment of weapon variants (IS has hundreds of competing companies trying to outdo each other, the Clans keep absorbing each other's technologies), but would have an easy time switching them out with an allotted amount of space for internal ammunition before it starts getting mounted on the exterior. I need to think on this in more detail. (One important thing for the I.S. that I feel is necessary is to maintain specific engine limits. A huge factor in mech customization is that the costs of a 300 engine for a 35 ton mech is a fraction of that for a 100 ton mech, and though they weigh the same the engine for this mech is not the same as for that mech and not compatible.)
Ferro Fibrous armor and Endo Steel structure may be lighter, but in the lore they are thicker and larger. If you look at the official scales for Clan Omnimechs you may notice that every mech with both endo steel and ferro fibrous armor are particularly large compared to those that lack endo steel and/or ferro fibrous. The Summoner 2 is supposedly a full .8 meters larger than the Summoner in just vertical height alone, then consider the larger girth it would also have. The large number of ferro fibrous builds in Clans means that their mechs will generally be larger than the IS mechs that lack either upgrade. Then the ones with Endo Steel compared to ones of the same weight without.. It'll make you thankful you don't have endo.
Anyway... seems I ranted on ideas. >.>
As for CW... check out the signature on the left.