This is not the most exciting topic, but it is an important one.
The system through which we generate our stars and planets will dictate the manner in which we can customize it and in which modders can extend it. Personally, I look for a manner to most effectively control a large number of variables through a relatively small number of switches and controls.
Our two contenders here are Nightfish and Drek, both of whom have gone to a lot of trouble to work on their proposals. They're fairly involved, but please read both of them completely before you vote or comment.
As always, this is not a thread to propose new systems. We had a DESIGN: thread about this on the old board, but since we don't have one here, if you desperately, absolutely must suggest another idea, PM me and I'll make a new design thread. But these are fairly comprehensive.
Drek's system: http://www.drektopia.com/startypes4.htm
(note: the section on specials is not married to the rest of the proposal, and since we aren't making a final call on specials now, it's just food for thought.)
Nightfish's system: http://www.drektopia.com/NFGalaxy2.htm
I'll withhold commentary on either for a little while.