Not sure how exactly they're chosen, hopefully someone will have an answer but this is how I think it should work.
1. Follow the same guidelines/restrictions for new ideas that we allready use
2. We have a month or so to post our ideas
3. Someone gathers all the data and statistics of how many people want what and so on...(I know this part would be time consuming)
4. We take the top 10 or 15 most wanted planes/vehicles and we are given a month to vote on, using a forum poll system.
5. Tally up the results and give HTC something a little more organized to work off of