The world is not built in a day, nor is a child board.
What typically is the MO of world builders that get their own forum is to start your threads like you are making them in your own forum and put the [World Name] in brackets like that in the subject line of the new thread you create.
You can create as many threads as you want. When you have a good chunk and it is moving forward, we create the board and gather your beautiful offspring into the fold and lay them gently there.
A second way to do this is in the main Citadel. We have a mysterious thing called "Regions" that are only visited by the most bravest warriors. Valadaar has been stomping around there lately and has found some issues which proves it is still in alpha/beta stages. But it needs more use in order to pull my attention towards with greater urgency. Are you the one that sounds the horn? Are you the one that joins valadaar in his mystic quest of exploring the regions?
In order to gain access to this long forgotten and dangerous location you must use the same freetext for all the subs that belong to that world. Upon the day when the Strolenati deem you worthy, the magic click of acceptance will gather your subs with freetext and compile them through the magic of Algor Ithm and plant them inside your chosen title. Henceforth, you will have your own region that gathers your glorious subs into a organization worthy of a librarian and gives a table of contents to those that visit.
So forums or regions. Both take some work to get and it sounds like you have a bottom up plan to work in the forum and promote them to the Citadel. A worthy plan and commend you for it!
So write away and once you get a few threads going you will be granted a forum of your own to live and work in.
(We did have a post count rule for it somewhere, just can't find the d**n thing so we will wing it. I would expect at least 5 different threads with a solid foundation of content. Otherwise I would have to give a new board to most everybody that has an idea for a world.)