Is Near Future Spacecraft permitted? It adds a few new command pods, engines, and fuel tanks, but other than that it doesn't do anything drastic. Sorry if this has already been answered :B
19 here, I try to find that sweet spot between cool-looking and completely functional, but it's admittedly not an easy task. If I can help it I try for the "used future" cobbled-together look with exposed girders and struts and things.