You largely end up with the same issues we have now with 6 man content. There is a tiny amount of LS across all servers that would be able to pull 18 people on the regular for an event, meaning applications/membership would probably shoot up as people flock to these LS for content. Then you end up with times where 24+ people show up for an 18 man event and people are excluded. a regular basis will be an organisers nightmare.
The reason why there aren't enough big LS these days is because 18 man content doesn't exist. If they exist then this kind of LS will start to pop as LS recruit more people for it. So it's not really an issue tbh.
Case to the point, when aeonic needed around 12-15+ people back in 2015 many endgame LS were mid-size LS. Once SMN burn is discovered the required manpower reduced to 6. Then the endgame community divided into many small groups SMN burning them. Then dyna wave 3 was introduced which requires an alliance again, and many endgame LS teamed up and recruit more people for such content at that time.
Of course now most LS is smaller because dyna wave 3 no longer require 6 people.
18 man content has it's perks from social pov. It gives big LS a reason to exist. There is one LS that I know of had a lot of people when dyna wave 3 was the main content, but now it's superrrrr small because everything is 6 man. So the better player in the LS forms their own clique and everyone else who can't get into the clique left. If the content requires 18 man, this wouldn't happen.
The problem about setup flexibility can easily be solved if
this 18man content doesn't require 18 man to beat.. If this 18 man content can be beaten by 12, and you have 24 people in the LS, you can simply create 2 groups of 12 and enter.
And when you have 16 people instead of 6 or 12 or 18, you can still put all 16 people together in the same group but no one is left out. Unlike 6 man content in this case will leave 4 people out of the group.
Some people may complain about leeching if 18 man content doesn't require 18 to beat. However since we already have hard content like Ody v25 I think having another leech-able easy content for the sake of putting the community together isn't a bad thing.