So before you go This again, let me tall you that this kind of implementation could probably give us the most of what we wanted from mechlab and could quell many of the fears that are present about mechlab.
The ideas is to create two different mechlabs: one limited version, that can be accessed in-game, when you enter a mech bay, and one that you can use to preset your variants, before you connect to a server (or even launch MWLL). Thought this may seem double the work for the same results (and it basically is), this can bring the best implementation of mechlab to MWLL. And IMO if I survived 2 years without mechlab, I can survive another 2 without it.
In-game mechlab:
This one would be a limited version, following a bit stricter rules than the one for MW4. Battlemechs will only be able to change their weapons within the same type (energy for energy, missile for missile, ballistic for ballistic) or remove them and change them for heatsinks (remove SRM2 for a single DHS,...). You will be unable to mount new equipment or to change your arm form energy to ballistic. You will also be unable to change the amount of armor and it's type, type of engine and engine rating or internal structure. Similar thing would apply for omnimechs but due to the nature of omnimechs, they would be able to exchange between types of weapons and add/remove electronics. Internal structure, Engine,... will still be limited. Though this will give Clan and omnimechs and advantage I recommend that because omnies require different supply lines than battlemechs, omnimechs only get this liberty at Main mech hangar and be unable to refit in single mechbays (in TC) or that refits at single mechbays cost additional C-Bills for omnimechs.
No one should be able to refit in field repair bays.
Standalone Mechlab:
This would be a totally free mechlab, the kinds you see in CBT. Everything is permitted (boating makes your mech cost skyrocket). This will allow us to see some exciting refits, but they should be priced accordingly (mounting a Hunchback mk.2 with 2x Gauss, Mounting MRM 20 on the arms of warhammer, Making atlas run at 80 kph for ultimate assault scout mech XD, quad gauss Fafnir

,...). The only limit would be your imagination, the in-game cost (you can create a mech that costs 400 000 C-bills, but will you ever be able to use it) and the number of variants you can take in-game (so we dont overflow the server with 12673421963493123 variants

, I would recommend around 10 variants per player, and everyone has access to stock variants).
What this does is enable the full refits of mechs and vehicles, but it also overcomes the problems we saw in MW4, where you could take 60 min (or more) to fully reconfigure your mech. If you need to quickly adapt to a situation in battlefield, you take one of your preset chassis, maybe change a weapon here and there and go, you don't spend 60 min: replacing the standard engine with XL, changing the ERPPC to LBX-10, removing 10 DHS for 15 HS, changing the 12 tons of STD armor to 14 tones of Stealth, adding LAMS and GECM, removing MASC and JJ and wondering if you should change that 1.5 tones spare you have to 2 tones for armor or should you add more armor, another weapon, or should you add AECM over GECM or maybe don't change that ERPPC at all and change...
P.S. When I say Battlemechs and omnimechs, the same goes for all the vehicles, not just mechs.