That's a great suggestion.
The current explosion option is currently pretty simple as it treats all objects equally (explosions are the same for planets or stars).
I have grand plans for continuing to add details like the one you suggested to Universe Sandbox. I've added it to the list.
Regarding your idea about the Big Bang (which is something I'd love to do, but not sure how) see this thread:
http://universesandbox.com/forum/index.php/topic,17.0.htmlRegarding the Big Rip:
http://en.wikipedia.org/wiki/Big_RipIt's not obvious to me how one would simulate the Big Rip. Sorry.