Plan out every detail. What furniture and where will it go? Light switches and sockets. Map out the floor plan for real in a space and use bits of cardboard to mimic the bed etc - this helps with working out how much room you will have a d which layout works. I did crazy stuff like miming how I get washed , dressed and made up to see how that flowed in the room, it sounds mad but does help you to design something that works well, rather than just looking pretty.
Yes to lots of light.
Think about windows - we have Velux on one side that can open from the top as well as swivelling in the middle - so much more air in the summer and you don't bash your head on them. Also the finishing of the velux is important - straight or angles recesses? How will the windows get cleaned?
En-suite - we designed ours so it has a separate door off the landing. This way we can let others use the bathroom too without having to come into our room, and we don't get any damp/condensation/poo smells in the bedroom.
Think about keeping a bit of accessible loft space for Xmas decs and suitcases etc.
Get as much ceiling height as you can. Consider getting the steels embedded between joists to get higher ceilings (wish we had known this!).
From the outside think of the symmetry of the windows - line them up with the first floor or it could look weird. Decide on the finish you want - we went s 'proper' wall rather than tiled and it looks so much better.
Size - we asked our builder to max it as much as poss. Our loft is a good metre deeper than our semi neighbours and that can make a huge difference.
Doors - think about pocket doors to save space and give clean lines.
Shelves and storage - as per the furniture plan think of this as part of the design and include in the building spec as it's likely to be cheaper that way.
Hope this helps.