Just an idea, but it would be nice to have an extended pre-ready time for reserved slots. It could be just a flat time that is longer than the current one, or a choose you're pre-ready time, or even a permanent pre-ready time. This would prevent someone in a reserved slot from going afk for a bit and then the lobby filling and subsequently falling apart because someone in a reserved slot dropped and didn't come back. This is primarily important for a lobby leader since if it is anyone else the leader can just un-reserve their slot.
You're probably thinking "but then they might not be back for the lobby once it starts" and you are correct, but they will automatically be !repped after a certain amount of time and a sub will come instead of the lobbying emptying and never filling.
Last weekend I finished a lobby, and since lobbies almost always take forever to fill because of Med/Heavy/Engie, I started a new lobby right away (of course reserving the slot I wanted) and went afk to step outside for a cig. When I got back inside I saw that the lobby had filled but since it was after my pre-ready timer ran out I did not ready-up and subsequently I got a lot of false accusations of being a troll and half the people left. We were left with a slowly filling lobby with 30 people in spectate that after 40 minutes I just closed because I was tired of waiting. Sure, if I had a perma pre-ready option that might have meant I would have been !repped and replaced had I not made it back in from my smoke in time, but at least everybody else would have gotten to start their lobby.