Let me preface before taking my own shot at this issue that I do not have, or could find, population data to back this up. These assumptions are simply based on my own experiences.
The question over whether or not to open a new world (or close/combine) focuses more on potential revenue than anything else. Potential revenue coming from existing coaches acquiring another team, relapsed coaches becoming active again, or new coaches getting their first team. These three avenues should be considered for any combination of opening or closing worlds. In my experience it seems that most coaches have teams in more than one world. That being said:
Open a new world
Existing Coaches - As pointed out by others many coaches will want to get into a new world for the chase to the top. Also pointed out by others, after that chase is over this is as unique as any other world out there.
Relapsed Coaches – I could see the possibility of marketing to them the same chase that would attract your multiple world coaches would attract back more than a few relapsed coaches. They clearly have a higher probability of relapsing after the chase is over.
New Coaches – Without a marketing effort to tell people unfamiliar with HD few brand new coaches will be acquired simply due to a new world being open. Past that there is a decent chance in my mind that with the initial few seasons having increased competition at DIII and DII from the influx of seasoned coaches from other worlds could backfire and drive off new coaches who find the learning curve too steep with the competition level.
Closing/Combining worlds
Existing Coaches – If my assumption of most (50%+) of coaches having multiple teams then there is a decent chance that in closing two worlds to form one new one that a small but engaged group of coaches will have teams in both worlds that now will only have one. This cuts down on revenue for this world vs two worlds. Additionally there is a small percentage of coaches who have been at one school for 50+ seasons who won’t want to start over at the same or different school having lost that history and nostalgia. And just like with opening a new world there will be the coaches who grab a team in this world just to be in the chase.
Relapsed Coaches – Little to no effect for relapsed coaches compared to a new world. Some will still become active again for the chase.
New Coaches – Little to no effect for new coaches compared to a new world.
Ghost Towns
I may be in the minority here but I do not think ghost towns are truly bad for the health of the game at the DI level. Part of what I enjoy of the current state of HD is that I can adjust my team/conference to the amount of time I have in my life to devote to HD. When I was in college I had multiple teams in 2x worlds with full conferences and loved that there was almost always recruiting going on. Then when I joined the work force I went down to one world in an empty conference. This allowed me to keep a high level of success with a fraction of the time that a single world in a full conference currently takes me. I think this is a great asset to the game as it stands that I am able to make HD work for my schedule instead of the other way around.
Odds of any Changes
Without knowing the fixed cost associated with opening and running a new world it is hard for anyone to say one way or another. Is the minimum average number of players to make a world breakeven 200? 300? How many coaches have already saturated their personal team limit? How much additional workload does a new world add to the staff? With all these questions I personally would rather see time, energy, and money go towards getting new coaches brought into the site as opposed to just offering a new world. Each new coach has the potential to get teams in multiple worlds where a new world only offers (primarily) existing coaches another opportunity at a team.