Giving one side some techs to use could work, especially if you don’t allow any new techs after that. I mean, if you give the “weaker” side heavy bombers then the “stronger” player goes and develops heavy bombers, you might be right back where you started.
In the PC version of Classic, there were several options you could choose to vary the game. One was called “Axis Advantage” which gave Jet Fighters to Germany and Super Submarines to Japan at the start of the game. You could try something like that. If you are playing the Allies, perhaps give USA Heavy Bombers, UK Radar and USSR Improved Mechanized Infantry or Advanced Artillery to help even things out.
Heck, if you wanted to extend this idea to the smaller nations, give Italy Improved Shipyards when playing the Axis. When playing the Allies, give ANZAC Long Range Aircraft. You could even give France Increased Factory Production so if/when they get liberated, they can already have an edge on building stuff. Heck, you could even give China Advanced Artillery so when they do get to buy artillery, each one will support 2 of their infantry. Wouldn’t that give Japan fits.