What I'd like is a "standard" probe type, that can be upgraded to different generations with about 80% of the R&D done after the first one in that type's generation: Flyby, Orbiter, lander, rover, sample return. Once you get a flyby probe in Generation 2, you can send the same model to Mars, Venus, Jupiter, Saturn, Neptune, etc. The concept comes from the "standard satellite" concept that NASA's toying with, in the CUBEsats among others in an effort to cut down R&D and production/launch costs.
And one thing that might make landing missions more realistic is to have a goose in the reliability marks for the manned landing the more unmanned landings you make - after all, the more unmanned missions you send, the better you know what you're sending your astronauts/cosmonauts into and what you need to design into their equipment to make an unavoidably dangerous mission more dangerous than it has to be.
(By the way, Ultimate Sandbox would be more than a great mode, it would be a great name. I'm thinking ofthis:
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