Below is the part I think could be implemented very well in MWO, with little or few changes needed:
Refer to Strategic Operations, p.188:
Quote
Retif Kits: Refit kits are factory-produced packages, often created by the unit's original manufacturer. The parts have been carefully selected to be compatible with a specific unit. Extensive supporting documentation is provided to make installation relatively straightforward. The disadvantage of refit kits is that they offer only a limited choice of options. Refit kits are graded by complexity. A relatively minor modification can be done in the field, but others are more involved and require a fully equipped maintenance facility or even access to the kind of support only found in a manufacturing center. (...) The classification of a refit kit is determined by the highest class of modification included within the kit.
Classes go from A to F, where A and B types can be done in the field, C and D require a maintenance facility while E and F require a full-blown factory.
I could very well see this implemented in the F2P model of MWO, where PGI controls the Refit Kits and their content, which 'Mech they can apply to and how much they cost. Installing a Refit Kit could depend on your location in the Inner Sphere (are you on a base with 'Mech factories, on a DropShip with limited maintenance facilities or in the field?). Installing the refit Kit could also depend on the skill of your Tech Crew, which would also impact the quality of the refit and its durability (?).
Then the rulebook talks about another sort of controlled customization, where you can go beyond the limitations of a refit Kit, but at greater cost, with greater install time, and with the side-effect of voiding your 'Mech's warranty (meaning you could lose the right to insure the 'Mech maybe?). I think this could also work as you can't just install anything anywhere with this system. It's still very restricted. The "upgrades" or customized refits could also be created and controlled by PGI.
Every item mentioned above could also be subject to availability rules (meaning some are rarer and wouldn't be purchasable from just any planet or faction).
...
And here is the part that I think can't possibly be implemented in MWO without having serious balance and tweaking issues: The "FrankenMech" rules.
In short, they basically allow you to piece together parts from different 'Mech models and weight class, and let you create "monsters". The implications and combinations are so infinite (and sometimes make no sense) that I think it would be impossible for PGI to properly implement this without having overpowered combinations or things that just can't be balanced.
(edit) I previously quoted a large portion of the rulebook, but due to IP, I decided (after being advised by a fellow member) to strip it down to this essential bit to avoid trouble. I hope it's okay now...
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Edited by Tweaks, 16 December 2011 - 09:23 PM.