Honestly, I'd rather see the more basic stuff like an in-program texture selection interface, being able to save things like rings and rotational values, and the aligning orbits with the parent's axis thing I mentioned in a previous thread.
It also might be neat to an object's pericentron and apocentron show up when you right click on it along with things like surface gravity.
Also, I agree with the previously submitted idea of adding a system exchange onto this site at some point.
a star lifespan
I dunno... It seems like if you tried to simulate a planetary system over time periods where star lifespan comes into effect, you'd have to set the time step so high everything would glitch out and any planets or companion stars the star had would go flying.
This, and things like the atmosphere and temperature calculations, seem like they're a bit complex to be added in the short term future, since there are so many variables. Anyway, I normally just use Google to find the equations I need to calculate things like luminosity.