Current 10m raider here...
I just think it'd be nice to be able to invite along the extra players we often have online to the raid on a given night. Instead, because we are stuck with either using 10 people or 25 people, that means if we have say, 11-14 people online that are competent raiders, 1-4 are going to have to sit. This is obviously not fun for those people if they are online and wanting to raid.
What I'd really love to see is the fights scaling dynamically in groups of five, so that you can have either a 10, 15, 20, or 25 man raid.