If there's going to be a big open solar system that everyone's sharing, interplanetary flight is going to be impractical... I think it could only work as something like Minecraft where people build their stations and bases and such and other people can just go check them out whenever they feel like it. I haven't played Minecraft but it seems like that system works well. For longer trips, though, I think you'd have to just agree to use one of the warp drive MFDs.
...Or is that what the OP is proposing? Like, something that plugs into Transx or IMFD and would put you in a certain spot in the trajectory relative to the body you selected as the destination, but without time accelerating the body? For instance, if I want to go check out someone's base on Ganymede, I'd set up the course like normal, so that my PeA at Ganymede is, say, 100k, then in this other MFD I'd press the "Closest Approach" button or select how many seconds into "my" future I wanted to go or whatever, and bang, there I'd be at 100k on that planned trajectory relative to Ganymede, but without moving the moon at all. Then I do my retrograde burn and land like normal.
I guess that's basically the same thing as just using the scenario editor, though, isn't it? Shoot, that might just be the way to do it. There's going to have to be some fudging somewhere.
For more specific, cooperative flights, like trying to race someone to Neptune or something, I think you'd have to set it up with someone beforehand in a separate "room" on the server. I haven't played games online in years, though, so maybe I'm completely out of date.
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One thing to try in the real time solar system might be an option to make ships "persistent," in that they stay doing what they're doing even when you're not logged in, and then you could check in on them like once a week or something (like if you've ever owned a cactus) playing faster Orbiter sessions in the mean time.
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