Interesting project.

1930s track bike/path racer
1940s french 650B?
1950s italian city bike

1960s track or french constructeur and 1970s definitely a road bike.

In the 1980s things get a little more difficult. A nice sport touring bike from the 1980s or touring bike would be a good fit. For me, personally, nothing defines the 1990s for me more than weird suspension and colourful anodized CNC mountain bike parts. Road bikes evolved through every material known in the 1990s so you could choose many iconic machines from that period, but still I would say nothing says 1990s like a hard tail MTB with a nice rockshock. I'd go for the earlier half personally but by the late 1990s disc brakes and better suspension were emerging. 2000s are tough, lots of things happened then, certainly the fixie craze was memorable. You could get an aerospoke and through it on a debernardi and call it done, world touring also enjoyed a surge in popularity and the roll-off became a big deal. A thorn touring machine with rolloff would be pretty appropriate.

I'll always have a modern mountain bike, never more than 3 years old so when this decade passes you may not want to still have that bike. A fat bike or other plus sized tire machine would be pretty iconic for this decade but so too would a modern steel road bike with disc brakes and electronic shifting. My road bike has caliper brakes and machnical sram shifting, I'd love the modern stuff, but for me the money is better spent on keeping a new full suspension mtb, there is way more performance to be gained with new suspension and 1x drivetrains or wider, lighter MTB wheels etc.