And continuing...
Beta testing! Listen to your testers don't release before it is ready. From my experience Shrapnel is pretty good about this but I want to make sure it stays fresh in your mind
Also I think hiring on some of the great se4 modders for the dev team would be a good idea. A few names come to mind and I can provide a list if it is wanted.
back to features
Make the game scale to different resolutions effortlessly. I run my desktop at 2048x1152 and se4 becomes quite the small little box(using SJ's windowed SE4 tool)
Descriptive error windows! nothing is more frustrating to have something go horribly wrong and never know what caused it(also useful for modders)
ease of use features are a must for late game play when you have 300+ planets and thousands of ships! Minimal clicks(something se5 failed horribly on and the main reason I don't play it)
Already been mentioned but a quality map/scenario editor is a must.
water planets and multiple friendly races have colonies on the same planet like one in the ocean and one on land(this goes along with total modability though)
Everyone on the team that has any say in how the stock gameplay works must play SuicideJunkie's Carrier Battles mod the poster child for balanced and fun gameplay
Ship the game with mods! alot of people still don't use websites and download things for some reason so make sure the game gets to modders early(like have them on the beta team) so they can have finished mods to ship on the game cd/dvd
PBW intergration: have the game automagically send/recieve turns from the PBW system so theres no file moving needed. Also this goes along with having the right people in the beta/dev team make sure the game is supported by PBW (
pbw.spaceempires.net) and make the standards open so anyone can make their own PBW implementation maybe even have an official site/server
hopefully still more to come once I gather more thoughts into words