I would suggest the following candidates if you wanted to take the total VCs on the AA50 board from 18 to 24.
Germany: NorthWestern Europe - VC Antwerp
Russia: Novosibirsk - VC Novosibirsk
Japan: FIC Thailand - VC Bangkok (or Singapore if you prefer, in 1942)
Britain: Egypt - VC Cairo
Italy: Libya - VC Tripoli
USA: Brazil - VC Rio de Janeiro
All reasonable as major city centers, and all strategically placed to make the gameplay more interesting.
This would increase each player nation by 1 extra city on an even split 3 and 3 (Italy finally gets a second VC under its control, with Cairo nearby as well.) This expansion can be adapted for the All VC Factories HR too (where all VCs have a starting factory, but no new factories can be purchased), to introduce 12 additional points of production onto the board if desired. The overall split at the start would be 15/9 in favor of allies, but with more than 3 VCs in range of Axis during the first round that would bring it to 12-12 after the opening. 24 total VCs would give a larger spread, more area to defend or attack, and more potential paths to victory for either side.
Basically the lowest threshold of victory would be 16 total (for the Allies to reclaim all their starting VCs and then take one more), but the spread could make Victory options more nuanced as well, and equal for both sides. Such as victory at 18, 20, 21 or 22. 22 is all VCs +1 capital conquered, which would be more or less a victory lap. The lower end 16-20 is the same for basically both sides. Above 21 would be more challenging for Axis (on account of needing W. USA or a capital) so basically Total Victory. But at 16, 18 and 20, you could conceivably have more narrow games that don’t require the capital capture dynamic to play out.