If introducing new units to the 1942.2 roster, I think you could use the opportunity to promote a better (or at least new and interesting) overall balance by sides in the process. But before diving too much into it, just a preface that I don’t believe the 1942.2 game map would support the G40 production profiles as a purchasable units, so I would stick to the OOB values/rules for the Factory unit. Trying to use the G40 system of Major and Minor industrial complexes would likely require a reworking of the game to be feasible and would allow many production exploits/limitations on a map at this scale. If you do want to change the production profiles, just to realize that this is a major overhaul, that will likely require a complete reworking of the game, or an alteration to the OOB 1940 rules. I find the later option less desirable, better to keep the same rules as G40 whenever possible, for ease of introduction.
With respect to bases, I’d share the same concern as others, that the movement advantage and ability to scramble would be too distorting for this map. A nerfed base with altered rules doesn’t seem worth the effort, so if you wish to introduce a repair mechanic for ships (e.g. the two hit carrier) this ability could perhaps be folded into the normal “coastal” factory unit?
Basically I’d just stick to an expansion for combat units and leave the facilities as is, for simplicity, to see how it alters the game. One easy way would be to simply add these 1 of each new units to the starting set up, rather than trying to switch out OOB units to incorporate new G40 ones.
For example, just give Russia one of each new unit type, as a way of introducing it:
Moscow: 1 Mech and 1 Tactical bomber.
This is equivalent to a 15 ipc bid for Allies, allows for better Russian opening (a less dicey W. Russia battle for example, or a better Ukraine opener etc.) So it at least in the opening round the introduction of the 2 new G40 unit types, would represent an initial boost to Russia (Allies), and then just let everyone else buy them. If that isn’t enough then give each nation 1 Mech and 1 Tactical bomber at one of their second Victory City. This would provide some new interest for all nations, but would still favor Allies overall, since there are 3 nations on the Allied team vs 2 Nations on the Axis team. That would be 1 Tac B and 1 mech at…
Leningrad: for a bolder Russian opening, and potential strike against sz 5, the mech could make it to Evenki, the TacB to Bury for KJF set ups, otherwise they could give Russia more punch on the Eastern Front.
Paris: less dangerous than locating them at Rome, but still providing some cool G1 interest. From her the TacB could be used either against sz 7, sz 14 or sz 17 to give the Germans a leg up. The mech can race pretty for towards the east, or perhaps be used in Africa.
Calcutta: more potent KJF opener or Africa opener, to counter the expanded Axis threat. The TacB would make sz 37 more viable, or at least give extra cover for sz 61, alternatively it could go to the Eastern Front or Egypt. Alternatively the Mech might would a hit on FIC workable.
Manila: this seems a bit less distorting than Shanghai, since the TacB would have to move out off an island, and the mech would have to be transported, still giving Japan something extra to work with but without being as overpowered on J1.
Honolulu: I’d vote for putting the extra units at Hawaii, just to encourage the dual front war. The logic being that with a few more units in this theater the likelihood, the American player is more likely to dedicated resources in the Pac.
Another way would be to give each player nation the two bonus units, 1 Mech and 1 Tac B for free, and allow them to determine the location using normal bid rules, or at a Victory city of their choosing.
Or you could try some alternative fixed set up change, but I think it’s easier to just make a simple addendum to the normal set up card. That way the starting unit set up change is as easy as possible to implement.
Despite my reservations, a nerfed base might be fun, if you can find rules you like that work for the map scale. One possible approach, if you want to pursue a full redesign of the game, would be to include them, but not as units for purchase. For example, if you wanted to try the bases, and G40 factories, then just lay it all out from the outset as permanent features of the map. This would prevent purchasing exploits, as you could just design all the facilities to work for your intended balance. So for example you might give Japan a minor factory in Manchuria to aid the overall balance, but Japan wouldn’t be able to just buy a ton of new minor factories all over the place, or drop a Major factory in Manchuria and push 10 units a round from there etc. Bases might work the same way, with set locations only, not as purchasable, that way you could build out the design around the units you want to introduce. Once you had a factory scheme worked out, you could use the actual TacB and Mech units, to re-balance the opener.