I'm thinking about this aswell, I would like my map to be on the server (need to test it somehow!)
I'm going to try to answer your question as good as i can:
How many resource nodes per team.
~8 for marines
~15 for aliens
How long till first door opens.
5mins
How long till siege door opens.
15-20 mins
Any idea's you might have.
Make it new and original. If you have some crazy idea, just run with it and see what comes up

Try new solutions for problems etc. Look at other maps to see how they do it and then do something totally different. A theme for the map might be good too. My theme is a nuclear reactor (ns_reactorsiege).