@FallenTrinityOkay, so, here's what I suggest.
Humanity has colonized the majority of the star system- there are bases and such on many of the moons, such as Europa or Titan, Mars has moderate colonization, the Moon/Luna is a bustling spaceport and trading hub, there are likely more than a couple large space stations in orbit around both Earth and Mars that cater both to military and commercial purposes. There are satellites and such in orbit around planets in other solar systems, likely with some automatic colonization packages sent along- things that hit the group, deploy some 'bots, and starts clearing and building up a small infrastructure for the first colony to use when they finally go. Maybe they're en route. Either way, I think this would be reasonable for their to be ACTUAL infrastructure in the solar system for this sort of time period. There'd likely be a population boom, and the population of the solar system might be around 12 billion by now, with the average lifespan probably extending to 120 or 130, with one's prime starting to end more into their forties than their thirties, and only becoming a 'senior' when they're about 70-80. They'll only really be frail at 100+.
That's what I'd suggest.
More than that, the UN's IDF probably wouldn't be very powerful. The fear of extraterrestrials would be present, most definitely, but when humanity has reverse engineered a fuckton of it and has spread through the stars, that initial fear would've faded greatly. In this, I'd suggest that the IDF has greatly faded, where people still sorta participate just to keep up the pretense. In actuality, I'd figure that major nations of similar minds would join together- the North Americas and maybe some European countries would probably make a major alliance/faction, such as America, Canada, Great Britain, France, and Germany- maybe even South Korea, Spain, Greece, etc. Another faction might be Russia, China, North Korea, and other similar states. A third, weaker faction might be places like Israel, Saudi Arabia, India, and Egypt.
These three factions would be, I figure, the major players. Feel free to edit it however you want, but these are the closest, whether geographically, politically, ideologically, etc.
Mechs would likely be Titan-sized, in my mind- about 30' or a little less. They'd be heavy enough to operate on places such as Mars, resilient enough to stand up to those elements, as well as to survive 'titanfall,' strong enough to take care of objectives quickly and efficiently, mobile enough to avoid heavy, damaging fire, powerful enough to keep themselves running. They'd likely rely more on shields than armor. If shells are hitting armor, then you're going to be shaken up and screwed up a bit, and every hit is a chance for disaster. Shields can regenerate, rely solely on energy, don't need to be repaired, and can take those hits without the chance of a critical hit, so to speak.
Weapons would be limited- likely nothing over 160mm, maximum. Railguns might be in the 20-40mm range, cannon might be up to 105mm, autocannon would be up to 40mm. Each mech would have a minimum of half-coverage light armor, a full life support suite, emergency ejection, a self-contained cockpit, and a communications suite. One fusion reactor would work best, I think, and would keep the entire thing running. If severely damaged, it might screw up and blow the entire area to hell, but then again, so would a nuclear reactor, and fusion has a helluva lot more energy production potential.
Each mech would probably be limited to two or three weapons- likely an autocannon or chaingun, and then a cannon or small railgun, and maybe a mortar tube or missile launcher. Chainguns would likely be 5mm to 15mm- 5mm would have higher rate of fire, laser-accurate, might have two of them mounted, and would be used against light armor or infantry. 15mm might be useful against structures, suppression of Titans, and medium armor. Ish. Missiles would probably be 60mm to 80mm, mortars might be 105mm to 160mm.
Energy weapons?
Some smaller/stealthier titans might have a sort-of plasma blade, that it can activate to use in the event of close combat. I don't like swords and shit, but activating a device that's on your mech's arm to cut down a rushing mech or the like would be useful. Self-defense rather than an offensive weapon. Plasma cannon or an ion cannon would be limited in use- it'd consume quite a bit of energy which might deprive other systems of it, and would overheat quickly- and lasers would be limited in effectiveness. Lasers, in all seriousness, would only be useful against aircraft, or something else that can be exposed to the laser for a 'long' time, without heat-resistant plating. Lasers aren't red beams of destruction, it's just a concentrated beam of heat. You could melt the walls of a fuel tank, the electronics, etc. Good against slow cruise missiles or something. Concentrated beams of ionized particles- the more traditional cut-through-everything lasers- would have limited use, but would likely be very accurate and good for making holes in things, or cutting up infantry.
As ever, energy weapons' effectiveness decreases with atmosphere. On Earth, where there's lots of dust and such in the air, a laser would travel much less distance than a laser on the moon. On Earth, that laser is constantly bouncing off particles in the air, while there's nothing on the moon. It ultimately wouldn't make much difference, though. The higher the power it has, however, the worse that damage dropoff will be.
In terms of equipment- if we're being launched like the titans from Titanfall, that means we're going to have orbital support, in some form or another. Even if our ship has to immediately leave, it can still map out the ENTIRE terrain and beam it to us, as well as leave small stealth satellites that can continue to monitor visually, with infrared, etc. These could also spot targets, laser-designate enemies and positions to get hit by mortar or cannon.
In addition to that- larger weapons CAN be deployed. Instead of firing off a capsule that deploys a Titan when it lands, there can also be capsules that turn into stationary anti-air platforms, or surveillance platforms. Artillery, missile, etc. Larger ones might be large railguns or cruise missile launch pads.
Hell, that could be our two primary objectives in our missions- in the first type of mission, we go in to eliminate a threat- a convoy, a base, a large group of hostiles, etc. Or, we're going into an area to secure, defend, and deploy these capsules- likely surveillance. If there is a large, heavily defended base, then rather than mount an assault, we could set up a surveillance, railgun, and area denial capsule just within range. We defend it from their mechs trying to disable us, while the area denial capsule fires off missiles or a small railgun that destroys enemy artillery/railguns, while the surveillance guides the railgun to fire on the required targets. Or maybe just aerial denial and surveillance- surveillance gets more accurate readings on targets, so that a ship in orbit can launch several kinetic packages to strike the enemy aerial denial platforms and major defensive positions, and then our mechs clean up the attacking forces, and then move onto their base and clean that up too.
