Frankly, I think Tripods and Superheavies would be easier for them to do as their own program...
Why? If SSW is built correctly these will just "extend" the existing classes. For example, in MegaMek VTOL extends Tank because it shares all of the Tank class's base properties except a few which it "overrides". That's one of the beautiful things about Java. Actually, the "Aero" tree is probably the best example of this in MegaMek.
Entity (Base Class) -> Aero (Extends Entity) -> Conventional Fighters, Small Craft, and Jumpship (All Extend Aero) -> Dropship (Extends Small Craft) & Warship (Extends Jumpship)
Extendable classes are one of the best things about Java. So, there really would be no reason to have
another designer when a simple extension of the current classes should be a fairly easy solution. Unless I'm badly mistaken this is exactly how MegaMek will implement Superheavy Mechs... by either extending the various Mech Classes, or adding to them as appropriate. Of course, Bee would need to say for sure what his plans are, I'm only guessing ;)
Still, code-wise SSW seems to be fairly intelligently developed, so I'm sure a similar approach will be made there. Though sometimes I wish the developers had developed for MegaMekLab instead of starting their own project. :)