[First time I've made a thread so please let me know if I need to do anything differently or post somewhere else ]
So here's a suggestion - functional simulated Space Elevators and Skyhooks not as vessels but as KSC pseudo-Structures that can be upgraded like any other. I don't know if I'll ever get around to making this myself so I want to put the concept out there for feedback and for anyone with more modding experience than me.
I'll start with Space Elevators since they're simpler to use and implement. Using one would go something like clicking on the structure in the KSC, seeing the same menu you get when you click on the Launchpad and Runway, selecting a vessel to "launch", and then loading to a floating "structure" in geostationary orbit above the KSC. We basically render the station at the top of the space elevator, not the elevator itself, so it just floats there acting as a launchpad. There could be multiple such structures at different altitudes up the length of the elevator, from LKO up to GEO, although you'd only find yourself in a circular orbit if you set off from the GEO point since you wouldn't be starting with enough orbital velocity. From your chosen launch point you detach and off you go. You can use this method to put entire spaceships (subject to mass limits determined by structure upgrade level) into orbit, or put individual modules of a ship or station into orbit one at a time and then dock them. You could even bring canisters of material kits or rocket parts up if you're using Extraplanetary Launchpads, up to a waiting shipyard, enabling you to construct HUGE ships very easily for some seriously cool late-game stuff. This is my reason for wanting this mod, I'm tired of launching from the KSC or from EL bases on the moon and I want ways to streamline the process while remaining grounded in realistic futurism.
For balance we can use the stock building upgrade system to incrementally increase how much mass the elevator can accommodate in one "launch" - you'd start with a few tons, and work your way up to tens or maybe hundreds. I don't think it should ever be unlimited as with the fully upgraded Launchpad and Runway. That would be too easy, you may as well just use Hyperedit at that point.
For optimal functionality with EL, we'd need some way to have at least one vessel "docked" to the orbital launch point, otherwise we'd have to manually ferry cargo to a nearby station every time we brought it up, largely negating the whole point of the mod. I'm not sure how achievable this is in KSP. If it can be done, I'd like any vessel docked to the elevator to be able to receive unlimited resources from the surface, for a price of Resource Amount * Resource Cost * Operating Cost per Unit of Mass. If we can't dock with the structure in any traditional way, perhaps we can treat the shipyard as being in an unlaunched state on one of several staging points. Ideally that would mean there's a main "Launchpad" from which vessels brought up by the elevator are deployed, surrounded by one or more dummy launchpads where vessels can be placed, used and recovered but are unable to launch. This could get buggy though. Still trying to think of an ideal solution.
Finally, if we wanted to make this REALLY awesome, it would be pretty great to place such pseudo-structures on any body in the Kerbin System and beyond. This could be used with out EL but it would be much more useful if you were using it as part of some serious extraplanetary shipbuilding or resource-moving infrastructure. This would entail a special part which, in the case of the Space Elevator, acts as a sort of "setup kit" which can be deployed on, say, the Mun to create a Space Elevator endpoint. It would need to be placed at the equator of a body to work and would take time and resources to complete setup. Upgrades to the new elevator would be managed through the right click part menu, but I think the best solution would be a dedicated menu at the KSC and Tracking Station where we can manage all our launch structures throughout the system in one list, without having to load in and visit them.
We could also create skyhooks in the same way. The idea with skyhooks is they're cheaper to operate and in many ways simpler to build than space elevators. These would require you to fly a spaceplane to a specific speed and altitude eastwards over the equator. A toolbar menu would then enable you to "attach" to the skyhook, which would teleport the craft to a specific orbit depending on the skyhook.It would be fun to make even more narrow criteria and try and intercept the skyhook at exactly the right time and place, but it wouldn't be much use if you wanted to use the thing regularly. It would be nice to attach only the cargo somehow, but I don't know how realistic that is or how we'd achieve that in the mod since KSP only sees one craft irrespective of what's considered "cargo". Conceivably this would work offworld as well.
Alright, that's it. I think what I've outlined here would be quite achievable, if I or anyone else gets around to making such a thing, and it would be tremendously useful and immersive for builds like mine where I'm spending most of my time in the late game playing with mods like Interstellar Extended and USI Colonization. I'd love to hear what the rest of you guys think