I'm in Tom Seaver, which is currently on season 3 after starting as a brand new world. It starts with the players assigned more or less randomly, with each team getting a roughly even mix of veterans, prospects, etc. of varying quality (mostly bad). There are lots of bad contracts (I had a 32-year-old with low-A caliber ratings making $1.7m) and the talent level seems to stay low for a few seasons after launch—
this guy is my ace, and he'd be a fringy #5 in an established world. Most of the prospects created at launch are pretty bad, and coaches are also terrible at the start, so it'll probably take several more seasons for Seaver to approach the quality of longstanding worlds. Those factors make it an interesting and unique challenge that I'm glad I took on. It can be frustrating at times—I'm used to having a steady flow of quality prospects ready to reinforce my ML team, and in Seaver I had to immediately chuck all my prospects onto the ML roster to fill the gaping holes left by the initial allotment, and then I badly botched my first draft on top of that, so I currently have no useful prospects above high-A. However, the general dearth of talent makes you have to reevaluate every player—sure, that pitcher with 58 vR would never make it past AA in my other world, but here his 94 control might make him playable, etc.
Seaver and Curt Flood were the first new-from-scratch worlds in quite a while, I think, and I'm not sure if there are any other ones planned. It's a shame, because it's the only chance you'll get to tackle that sort of challenge.