Another issue (that will become a MAJOR pain in the arse) is HOW do you replace booted/kicked players?
Do you rely on players that just happen to be cruising the in-progress games OR do you automatically FORCE queued players into those vacancies?
Personally, I'd like the choice to start a new mission than jump into a session with a blown secondary.
I'm perfectly happy to keep on trucking with a group of 4 people. I am not happy however to run around back and forth, doing all the work while this leecher just sits there. IMO, a 4 person group is better than a 5 person group with a leecher.
If they do want to place the vacant spot with a new player. Maybe they can set a time limit. Say after 10 mins into the event, there will be no new replacements. I've jumped into an ISE that's already gone to hell plenty of times. I've always stayed and finished them, sometimes we even make the optional.
Honestly, I'm not too concerned about a vacant spot. If 3 people are competent in a group, they can pick up the slack for the rest. I just want the eye sore out of there.