etlegacy VS MechEngineer

You could try Dirty Bomb or the classic Wolfenstein: Enemy Territory for some class-based objective shooters. Both are free to play.

ET Legacy Wolfenstein Enemy Territory, FOSS client. Maybe moot since an official client re-released on Steam recently.

It doesn't replace any server, it's a new one on Legacy mod - https://www.etlegacy.com/

Look up ETLegacy some nice guys from the old scene have basically remastered it - hd graphics updated sounds etc. All things in one place!

ET: Legacy

Have you tried using ET Legacy? www.etlegacy.com It's basically the same ET client, but updated to current standards.

As you pointed you need source code that can be found here: https://github.com/etlegacy/etlegacy

Off topic, but if it was Wolfenstein ET I will just add that there still is a community for it and a Foss version of it that runs on modern operating systems

https://www.etlegacy.com/

You could just use Mech Engineer on its own, but even that recommends RogueTech or BTA so that you will have ready access to all the components that you need.

You might want to pick up one of the mods that includes Mech Engineer for greater tinkering. As the description mentions, both Battletech Advanced (BTA3062) and RogueTech include it, with the latter being more significantly different from vanilla. Lots more customization possible, and both of those mods include a lot for stuff to customize (RT moreso, like... 2600+ mechs/variants, vs ~1800 in BTA).

I can heartily recommend BattleTech Advanced if you don't want to go as far as Roguetech. It's definitely tougher, but not as impenetrable as Roguetech is. (Plus there's lots of difficulty sliders so you can probably tweak it to be about as forgiving as vanilla).

I really enjoy the MWO-style mech customization that both of these add (the https://github.com/BattletechModders/MechEngineer mod); one critique I have of vanilla Battletech is that they simplified the mech construction mechanics a lot, and stripped out some of the tradeoffs that make mech design interesting in the canon. In vanilla, you just optimize for free tonnage, and can put jump jets on everything, so there are just a few chassis in each weight class that are strictly optimal for most loadouts; this makes it quite boring as there's little reason to pick other chassis. In BTA/Roguetech you can take an XL engine to shave off a lot of weight, but that means your mech will die if you lose your side torso. Lots of meaningful trade-offs, with no "best solution".

I will admit that my eyes glossed over when I first started the game and looked at all the new part types I had to learn though...