I'd vote for whole chassis, due to convenience and also because it makes more sense, given that someone would have to in-setting have developed, tested, and produced said parts.

I honestly think some hybrid system might be nice, but one that heavily favors standardization over customization (higher cost, knowledge, effort). You can always add some variants of main chassis to spice things up if needed.

Sort of like how customization works in BT:

Is it possible to  customize a mech? Sure. 

Is it common or something the average soldier/mercenary has the benefit of? No, unless you count field mods or repairs, because you can't just haphazardly throw parts onto a mech and expect that it will work well.

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[quote=@Aristo]
[@Sophrus][@Gun][@Vigfast]
I'll try to be specific here; the pace of combat is intended to be slower and more deliberate than Armored Core or Gundam, but faster than Battletech/Mechwarrior. Imagine an infantry fireteam sweeping through the hallways of a building. Now imagine FRAMEs sweeping through urban streets. FRAME combat has the potential to be faster, as evident by the 'skate' ability they have, but you won't see many pilots zipping around everywhere in the middle of a firefight (unless they're dashing to cover or making a hit-and-run attack, perhaps). 

Here are a couple more cinematics to take reference from. Combat in the RP will probably be more deliberate and won't feature as much skating than they depict, but otherwise, they're pretty similar. FRAMEs are pretty mobile and are known for hitting quickly and hard.

https://youtu.be/WOL5NvSQffM
https://youtu.be/FNu5oFAsBIU
 
That said, I'm very flexible, so if you guys have common preferences, I'm willing to cater to them.
[/quote]

Ok, that sounds promising, I guess what I am always leery of is mechs that act more like giant humans (Gundam or Armored Core) rather than like machines.