I like this idea, but I suspect the majority of people drawn to your community would be more likely to be visual artists, architects, interior decorators, urban planners, and (especially) politicians/political philosophers rather than traditional worldbuilding hobbyists.
The idea of getting to decide everything about a city and see it magically appear is very different from the experience of sitting around drawing a map and figuring out linguistic drift for the fun of it. Just being able to make a city be whatever you want without rules whatsoever has a way broader appeal than what writers and worldbuilding hobbyists do.
If you've seen Inception, think of the scene where the architecture student wanders around making her own personal city in a dream, breaking all the laws of physics in the process. Almost anyone with any sort of interest in the visual arts would enjoy that kind of canvas without limitations. Worldbuilding hobbists, though, are often the opposite of that. We like playing within restrictions just as much as we like crazy flights of fantasy. We like the intellectual game of making things plausible using real-world sciences. 349
We enjoy the element of it that's like solving a puzzle, love it when our own creations surprise us, and live for that moment where we think logically about something we made up and discover something new about it ("Aha! Of course they'd have an economy based around glass!").
If you are imagining a world where there are absolutely no limited resources required to engage in these magical worldbuilding activities -- where land and magic are unlimited, so your average hobbist can give this new occupation a try -- I could imagine old-fashioned worldbuilders inventing games where they designed, say, physics systems and then tried to guess their outcome of those systems or worldbuild within those sets of rules. It could also imagine some people wanting to get together and enjoy the social experience of collaborating on a city or world, but that's not exactly the same thing, and I think many people in the former group wouldn't be automatically interested in the collaborative element. Both are in a sense forms of constrained writing, but the collaborative part would presumably be more likely to draw, say, people who like improv, or people who play story-heavy roleplaying games.
On the other hand, if there are resource limitations, I wouldn't expect the attitudes of worldbuilding hobbists to be heavilly represented in the worldbuilding field, because then in would be more a matter of money and politics, especially if real people are going to live in these invented places. At that point it stops being unlike the real world, which is already full of people building stuff with others.
Last but not least: consider the fact that a lot of worldbuilders are less interested in inventing landmasses and decorating palaces than inventing (and rationalizing) entire cultures, including things like religions and traditional table settings and the outcomes of wars. For those people, you would need for it to be possible for this magic to create something like AI citizens for this activity to be really satisfying. Otherwise I think you'd find a lot of worldbuilders right whether they are now: writing works of fiction on paper, where they can control everything except the things they prefer not to control. 159