it's essentially about coming up with something that has the right dV to be the same difficulty, we could take a look at it if there's demand
I have a concrete proposal.
Original Mercury Rocket in default game world with "Normal" game difficulty is making exactly 3.601 km of ΔV.
Realistic game difficulty have 20 times more distances and sizes, which requires exactly 20^0.5 = 4.47 times more ΔV for performing the same operations in comparison to the same performance in "Normal" world with 1:20 distances.
This means that we can simply multiply 3.601 by 4.47 and get the number 16.104 km of required ΔV for Realistic.
Original Mercury Rocket itself at Realistic level already can make 6.696 km of ΔV because of x1.5 ISP bonus, much lighter fuel tanks, and little more fuel in those tanks all combined. So we lack only 16.104 - 6.696 = 9.408 km of ΔV.
Thus we can simply add second stage to Original Mercury Rocket, which can make additional +9.408 km of ΔV.
And those two stages - Original Mercury Rocket plus second stage connected to it, orbiting Earth at low orbit above atmosphere (for example at 115 km above the ground) - will be start of the mission.
For example, it can look like this:
This second stage can do only 9.399 km of ΔV. It is simple and convenient. And the entire mass of this complex is 372.69 t.
But since it will not be possible to bring this entire complex in one launch from Earth - it must be connected by sections somehow. And connected parts can be dumped during the way to gain little more ΔV.
But connectors also have weight, so making fuel tank parts too small would lose ΔV instead.
For example, "Multi second stage" in the picture below can do 8.978 km of ΔV and it weighs only 296.11 t.
So it allows you to make very simple and convenient rules like this:
At Realistic level the starting rocket have to be in LEO at 115/115 km above the ground, and it must have the exactly Original Mercury Rocket plus any kind of separate second+third++ stages as additional propulsion system, with any configuration by players choice, but entire weight of all starting rocket with all stages must be no more than 300 t.
And people can assemble any formation they like, with full comfort, but they still will get no more than +9 km of ΔV from all additional stages.
You can allow more mass, like 320 t, if you wish. For example, this rocket below weighs 326.49 t, but can make only +9.378 km of ΔV.
So it is hard to imagine, that somebody can produce more than +9.408 of ΔV with 320 t limitation. And even if somebody will find a way to squeeze little more meters - then let it be, let's appreciate their constructional skills and let them have this little bonus reward then.
Thus in addition to original challenge itself, you also improve it by allowing players to exercise some creativity in designing all additional stages and connecting it in LEO from several launches from Earth. And also this will force them to be much better navigators if they made very inefficient design of their rocket, since they have to complete the mission with less ΔV in their hands.
So, what do you think about all that?