I would like the option to stop the countdown to mission start, as the host. I have encountered an issue where players with very low levels (<10) will join my party and run straight for the portal without waiting for a full team. Once the timer has started I cannot switch the mission or challenge rating. I have found that these low level players bail after the first wipe.

While I am happy to help players level up, I would choose a more appropriate challenge level based on the party.

It is also my experience that players will not join if a mission has not been selected, so waiting to select a mission until I have a party isn't the best option.