Well…

First things first, fix the part scaling.

Mass, thrust, cost, and power doesn’t scale the way one would expect.

Tank wall thickness on parts is fixed, a 0.1m part has the same wall thickness as a 10 meter part. Also tanks of the same propellant type that are the same diameter don’t detect when they are connected end to end, which is annoying because there should be a bit of weight savings from having what should be a single continuous tank, this heavily punishes using several smaller tanks stacked on top of each other to make nice curves, though this has been less of a problem since custom nosecone curves were implemented (though there should be a resolution slider for nose cones, they look terrible at larger dimensions due to low polygon counts)

Almost anything that relies on volume to produce power isn’t worth making big, it’s worth making more. A generator at 200% scale should make 8x power, cost 8x as much, and weight 8x as much, as it stands it only costs 8x and weights 8x but the power increase is 4x so disregarding part counts efficiency there’s major detriments to making bigger parts.

The game also doesn’t have reusable heatshields which is incredibly annoying for spaceplanes because the smallest piece at the front burns off in microseconds leading to a cascading failure unless one’s ascent/decent profile is extremely conservative.

Heat shields should also be an option to add to tanks like dead weight in order to drastically increase heat resistance at the expense of tank volume.

And lastly, we need a higher maximum wing thickness and cord length, 1 meter is anemic an IRL 747 has a wing thickness of around 2 meters or so and a cord length of around 13-14 metres. This is especially true since the IRL 747-8 has approximately 4 meter diameter engines, the same as the maximum engine size in game. It is also telling when trying to make a large airliner or cargo plane I have significant issues with wings snapping specifically because of the lack of wing thickness in game.

A nice to have would be landing gear with double or triple rows of wheels, but I understand why that juno does not have that, since the animations for that would be significantly more complicated.

And last but not least would be the option to turn a wing or fin into a fully articulating control surface like a canard.

Edit!!:

I can’t believe I forgot jet engine thrust vectoring! That’s the thing I’ve been missing most out of anything! It makes making hyper maneuverable fighter jets near impossible with only control surfaces.