A few folks and I are starting an alternate setting AM pbp game called Tales of Crimson and Gold. As a tongue in cheek concept it's Rifts for Ars Magica, and I am fully expecting a kitchen-sink hodgepodge of conflicting themes and disparate power levels. Have a read if you want to see the details of what has changed in the setting (TLDR = almost everything cannon is now irrelevant if we choose).
I'm not sure what the stories will be yet, but I'm anticipating many of the problems outlined in this discussion to be very prevalent for us. As the SG I'm hoping to work through them as needed.
(a) Effort in setting prep - I cant create a holistic world setting, flexible magic system, all the npcs, etc. Nobody has that time, and collaboration rarely works well unless a few people carry the primary work. Instead I'll write up what I need to to frame a story, and suss the rest in play at the time of posting. That approach is the only way to write when you are time limited. Iknow it will eb frustrating for the players, which is why the initial players are from my normal face to face table.
That means that the players and SG both have an obligation to accept the rough spots, contribute material, and accept that the contribution of others may not be what I had in mind.
(b) A game can't be ad-hoc and also say "that is not allowed" too often. I am accepting of this, but also worried by it. Could be a disaster, and certainly will be for any person who quotes rules and settings, or any player who desires a detailed backdrop. Going with the flow might bend the setting a lot, but it is already deliberately bend.
A commercially published setting does not have this degree of flexibility, because it will be and should be viewed as a product. I'd dearly love to write an alternate setting, but my job and family come first. I'd buy the Pdf if it was published too. Creation of an alternate setting book, akin to one of the dnd style world books or Rifts expansion setting books is what I can picture, and from my novice rpg publishing experience it is a monster project for a small audience.
(c) Power differences and the agency of the PCs. I'm openly allowing the players to write up and play whatever they like, up to a moderate power magus. So far we have two drafted companions who are moderate, and a few magi who are far more powerful by comparison. To challenge them I intend to use the strengths of the pbp format; (1) focus on the motivations and challenges to their view of the worlds, and (2) keep them facing their own challenges without always having the entire "party" present. A pbp game can run concurrent sessions better than a tabletop game, and a minute can be a lifetime.
(d) changes to magic, power, etc - I have not altered much in the plan, except for the removal of Parma Magica. While the thread has a great view on the PM being a factor in power level control, I am interested to see how everyone behaves when nobody in play has the protection of Parma. Beast still have MR, but the ability was never published by Bonisagus. It is an experiment.
This thread discussion is darn useful, and while I love the high fantasy aspect which can be played for Ars - I was one of the responders in the poll a while ago who said "keep it low fantasy mythic for core material please". This is because the game has a fan base who continually contribute, continually purchase books, and provide a tiny but exciting and evocative forum.
As most of the participants here already like how 5e is approaching the game, I think moving too far away from that, or a setting change, or trying to force a 6e too early will potentially alienate a large segment of players. For a start my investment in the 5e range of products is very large. It is the same reason that I am highly skeptical of the next iteration of DnD - why buy a product which is almost the same, but different? I've seen folks recently play 4e with mods very happily.
How much material would be present if D.C. and a few of the other primary contributors left, or didn't contribute to the alternate setting? (predisposing that that would, which is drawing a long bow) I'd guess not much. And highly opinionated people like myself do not have the time to finish a blogpost, so certainly cannot regularly create a finished book each season. That might also be seen to imply that the authors have a kind of ransom on the themes in 5e, but I prefer to think of that as a strong creative influence instead, and remember that it is a trust that has been earned many times over. I'm still trying to finish a part of the Vanilla Covenants Project, and shamefully it won't be soon. I love the VCP idea, it's just taking a boat load of time.
If an alternate setting was drafted or setup to accept contributing ideas I'd very happily pipe in with suggestions, but couldn't do much more than that. Happy for whatever madness in out little pbp stories in ToCG to be borrowed and used if they are entertaining, but I think the product line needs to stick to it's strengths and go forward.