As Kelvin said, a great suggestion Thurstain and I think Alex (TEQ80) was the 1st to attempt a trip this way with his upcoming Overnight snow trip. Noticed that Kelvin did the same with his Mt Cole trip asking for 'Express Interest Only', but over 20 have joined.
Playing the 'devils advocate', I see an issue with your 'No.3'. Just not sure how the Trip Leader can police those who expressed interest having 1st preference under the current system.... Do you send a 'PM" to all those who expressed interest advising that trip date is now locked in; meaning initially setting trip as 'private' for the 1st 12-24 hrs..? Reason I ask is that so many people express interest in trips, some on almost every trip that is posted, but when a spot becomes available, never seem to take it. Not sure if the interested is; " I'd like to take part in that trip", or "it sounds like an interesting trip, keen to hear how it goes".
Again, a good idea and well done Alex (TEQ80) giving Thurstain the idea of making this a future feature on the site along with Kelvin for giving it the 'green light' and no doubt having him doing the hard work to do the 'computer stuff' and set it up for the site.