You guys are simply going beyond the multiplayer behaviour capabilities of construct 2.
The official multiplayer system is a simple signaling server which connects players with each other, so one player will always be the host.
What you guys are talking about is a simple dedicated server, that's it.
So as long as you're not going to code a dedicated server, there's no way you're ever going to achieve what you're looking for.
From what i've read from Ashley so far... he's not really interested into further developing the multiplayer part.
It looked like he felt that the multiplayer feature was a complete waste of time, because there are not enough people actualy using it. (i'm just interpreting what i've read so far, but his words came pretty close to that) So according to that i would guess that he's not willing to help you in a dedicated server matter.
(i would love to get my hands on a dedicated server version as well)
The only way to prevent players to become host right now is, to run the "main game" on your own server.
What i mean is.... run the game itself on your own private dedicated windows server (for example) which will then always be the host.
That would prevent players to become host themselfs, and you could offer your players a dedicated bandwith.
But it's just a simple and bad workaround.
So either you're going with lennaerts suggestion, which is actually pretty good.
Or you're going to code your own dedicated server.
Using lennaerts suggestion would simply mean that creating an actual (dungeon) instance will just be a new player being abused as a new host.
This would save you an *** full of money rather than being forced to host a dedicated server.
Of course there's a big downside on that, as you would have to rely on the players internet connection/location.
But i'm sure you would be able to add some checks on the players connection to prevent such players to become a host.
If you're actually willing to go for a real MMORPG, just keep in mind that this will require a really fast and big database / a good dedicated server. MMORPGs require the most powerfull servers when it comes to gaming (compared to a shooter for example)
In short: it's expensive