I have some extended holiday time off, so I decided to try out the mods for this. TL

R - I rec trying out the Battletech Advanced 3062 (BTA) mod.
I never bought the 3 DLC's originally, so I went directly from base game -> 3 DLC + mods. So exactly what was added per the DLC's and what was added per the mods, I'm not always sure (which I know, isn't the most helpful).
I started with the seemingly most popular mod - RogueTech. I played one game and uninstalled this. The level of detail both in building mechs and shit shown in combat was just... overwhelming to say the least. I wanted more options - more mech, weps, etc - but this went WAY overboard for what I was looking for.
So I tried the 2nd most popular - Battletech Advanced 3062 (BTA). Much better for me. It was described as the mod that branched off from RogueTech once the bloat started getting out of control. Loving it so far. Sooooo many more mechs. Sooooo much more tech/items for mechs. Added a few additional pilot skills and tweaked the existing ones (no more Called Shot, which I'm ok with as it got broken once you get good weapons/items/skills), pilot bonuses based upon combat experience, etc etc. Also really digging the extra Argo upgrades - you can eventually have 2 full lances of mechs and a lance of vehicles under your control, per map. Everything was expanded in ways I liked, without going into autistic levels of detail. Really liking the rebalancing as well.
Combat is substantially better, and by that I mean the AI is fucking vicious. I love it, but damn, it took a minute to get used to. If a mech can get behind you to shoot your back - they will. Every time. They also seem to melee more when it's a good option (and there's much better options both for available mechs and mods/weps for melee). This increased difficulty has made the starting game slow as shit, as I'm literally clawing and scraping just to make monthly upkeep, while trying to upgrade my mechs, pilots, and the Argo. I was losing a lot of time and money both in repairing mechs and letting mechwarriors heal up, because even the half star missions will tear your shit up, as a new player. Man, I really forgot how gimpy you are with a base, un-upgraded Argo. And green pilots with shit skills. One of the starting careers gives you a nice upfront sum of credits, and it may be the best option. It's not the best longterm, but you have to GET to longterm for that to matter, so /shrug I'm sure if I wasn't trying to keep pretty much every partial mech piece my cash flow would be better. But... gotta catch 'em all! I have it set to take 4 salvage pieces to reform a mech, and even with the system allowing you to often use variations of mechs together to build one, due to the mass amount of different mechs in the game, it really takes a minute to get enough pieces to reform mechs.
Another nice feature - light mechs are now viable, or at least much much more than they were, as it's a bit easier to get evasion pips, and more importantly, you DO NOT lose one each time someone shoots you. Whatever you acquire from moving/skills/items, you keep until the next turn. This means small fast mechs can legit survive, but they are of course still going to bite it to a lucky 2-3 hits. And this evasion seems to have a lessened effect against melee, because you absolutely do not want to get these little guys in melee range of the opfor. I have a Locust with 2x ER ML and I love it for harassing and distracting the opfor, but he typically gets 6-8 pips per move.
There's a ton of other features I haven't even run across yet - like a hidden Super Heavy mech (~200 tons in lore, not sure in this game) you can only get pieces for from one mission from one faction, hero mechs earned from Solaris wins, etc, but there's so much more "game" here than there used to be, so if you like the game but wanted more, I would look at these mods.