I also think you should seriously limit the sim city aspect of the mod and concentrate on the fighting. There are several reasons for this. First, setting too lofty a goal is just setting yourself up for failure. Look at 90% of those vaporware mmo's (lord knows I've followed or betaed enough of them). Then, take for example the Forgotten Hope mod for BF/BF2. In 2 they've really zeroed in on the Afrika Korps campaign so that they can actually get something complete, done well, and out the door in a reasonable amount of time. Second, what is Battletech *REALLY* about, especially from the perspective of the average person's familiarity with the universe? The fighting. If you can nail that, and do it well, then you can spend the time to expand from there.
Now I'll narrow down to some specific suggestions / ideas.
Focus on the planet fighting, at least to begin with. While the conquest of galactic conquest is very cool, fighting in those little fat cross's that represent a planet definitely isn't. If at some point down the line it actually is possible to implement a tactical overhead map (ala MOO2 or something) I'd say go for it. As it stands, if you start with a single planetary battle style land based map, you have a great focused game. Someone suggested syncing units between the two play styles, I say if you have to have the 2 dif game types then this is an absolute must. Competing mods is just wasted resources IMO.
So for the planetary battles... Instead of having cities, make it so you build (settle) control points. The catch is, have several different kinds of control points. Make mech bays, garrisons, mines, power plants, farms... that kind of stuff. There may be some way for each type to yield a certain type of bonus, or allow production only at specific bases, etc. From there instead of building city improvements in each base, you can allow there to be built a very small number of improvements, or addons really. IE, build the 1.21 jiggawatt hyper thermonuclear capacitor addon for you powerplant base.
Culture... eliminated obviously but not completely useless. Call it Control, and the borders obviously mean nothing, however since civ tracks the amount of control you've produced you can probably tool this to allow for unit population caps, or addons or something to that effect, or unlocking specific units/addons. obviously science would have a similar effect so you'd have to settle on which does what, but you get the idea...
I've begun rambling and hopping topics a bit so i'll come back to this more later.
Too bad i'm not able to help much, only betatest, if needed 
![[pissed]](/images/smilies/pissed.gif "Pissed [pissed]")
