Space program eras

[Cassel]

When choosing the game mode: career, science, sandbox, I would also add the choice of the era of the space program, in which the player wants to play the game.

An early era would be what we have now.

The middle era is new parts, more efficient engines, more solar panels, some drilling equipment (eg for mining water and other resources), more structural parts and several modules for space stations. An addition would be a fixed base on the moon, from which the player could start newly created vehicles. Some vehicles would take off from the ground base, and others from Gateway in the orbit of the moon.
In career mode there would be new contracts for the lunar base, get 100 liters of water and deliver to the base or orbital station in a few hours. Or design a reusable lander that allows you to transport your crew or supplies from the Gateway to the moon base, and any use of this lander must be within the costs specified in the contract.

The late era is large ion engines, large nuclear reactors that allow vessels to be supplied far from the star, larger Nerv engines. A fixed base on the moon, slightly larger than that from the middle era and a small fixed base on the Duna.
If there will be life support in KSP, in this era we could get parts responsible for the production of food in orbit or terrestrial bases, and earlier eras would only have tanks with oxygen/food or parts that enable recycling.

[Gargamel]

I know the most hated answer for this forum is.... 'there's a mod for that', but...

I won't argue this isn't a good idea for stock, it is, but I don't see it happening.   That much work is into the DLC territory, and they won't make a DLC that is easily reproducible with mods, as it would kill the sales.   

You can set this up now, with careful application of mods.