Hardpoints. And size restrictions, but those are easy. Mostly hardpoints. So that you need to put missiles in your Catapult C1's arms. Other variants of the chassis, where they exist, are distinct chassis from the default. So if you find a K2, it's a different chassis than the C1, and they're not interchangeable.
In the single-player campaign, you will need to spend time, money, and have MechTechs and a MechBay available in order to customize a 'Mech.
(to retain 'Mech flavor): avoidance of degenerate gameplay. 10xMG is degenerate, but so is the typical alpha-striking laser boat. I don't necessarily want to enforce the flavor at the expense of personal creativity and strategy, but I want to keep it within a reasonable 'feels-like-BT-lore' space.
Q:
"Will parts have modifiers based on where they were found? (Poor Quality, Good Quality, etc? Could be described as weapon/location quirks?)"
A: Parts will have different manufacturers, at the very least.
The exact nature of 'Mech customization is still being worked on, so unfortunately there are no details to share just yet. I can say, however, that we are using the MWO weapon models, so while nothing's final I wouldn't expect weapon hardpoints to stray too far from what's available there.
We're not using MWO's hardpoint system. What
HBS_TopazOne said was that you shouldn't expect us to stray too far from what's
available with it. E.G. you won't be able to make a Marauder with 5 PPCs on one arm, simply because we don't have the assets to do that. We still haven't decided how to implement the mech customization system beyond "What assets do we have from MWO, and how does that limit us in terms of actual mech geometry?" My
personal opinion on engines is that you shouldn't be able to swap engines.
"I hope you at least include all the era-appropriate variants so we have lots to play with."
That's definitely part of the plan.
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The Beagle Active Probe and Guardian ECM systems are
incredibly powerful, and also incredibly binary. If those are what you consider electronic warfare equipment, then the chances of any player getting their hands on them are pretty small, yeah. I'm not even sure if I feel like including them in the game at all.
If, however, your notion of electronics warfare includes equipment to extend your detection range for blips, or spoof what weight class your mech is until people have LOS, et cetera... Then yes, that's gear that exists. Without getting too much into it, it's gear that specializes a given mech toward a given style of play.