I attempted something like that for a bit, where the RP was all world building.  It was probably a poor execution on the whole, but overall I was surprised that in such world building loving community an RP where all you do is world build got little attention.  Perhaps it was the poor execution.

However i'm not going to entertain any new sci-fi NRPs for the time being since there's enough of them currently.  Anymore and I worry that the player base will get too stretched out which also leads to RP death.

Another issue with sci-fi NRPs is that there's no real conventions so to speak.  Fantasy RPs for instance don't have problems with scale or technology in the way that say, a sci-fi RP might.  A GM of a fantasy RP can state a time period they want tech to be inspired from and you pretty much are complete.  Scaling in fantasy RPs also tends to be easier since anyone with knowledge of history will know what "scale" works for what level of technology.  A bronze age RP can take place around a single region, a Iron Age RP across a sub-continent, a industrial RP across a planet.  

But in sci-fi, technology and scaling get very problematic especially in the free form space operas.  You will get people with tech beyond everyone else or people who decide to "be clever" and deliberately make tech that fucks everyone else over or people whose idea of advanced clash hard with others.  Let's say in a space NRP I said "This is a NRP with 23rd century tech".

Player 1 decides that means some nice sturdy power suits and gauss.

Player 2 decides that means having swarms of AI controlled von neumann probes with uploaded consciousnesses making use of particle weapons to wipe everyone out.

Than Player 3 decides it means you get a spaceship with a million stats attached to it powered by technobabble.

And than player 4 asks "may I have Europa based aliens with quantum mindtech that makes people's heads melt?  They'll be balanced I promise."

While this probably is somewhat of a problem for fantasy NRPs, it's simply not nearly to the extent i've seen it in sci-fi NRPs.  Especially the advanced ones.

The casual ones due to lack of standards mean balance is achieved because most people will be inevitably powerplaying in some shape or form.