Sorry, you responded while I was out on vacation, and by the time I got back it was apparently pretty buried by other threads.
In terms of having the shared progress, that would be very difficult to do; I'm not sure that I can think of a way to make that really work all that well without literally giving you double resources when you're adventuring together. I suppose we could make something like a rule where anyone not on the server gets a copy of resources that are collected or something, but in a server with many people that would really give them quite a many duplicate copies. That's something we could make a server option, sure, but still.
In terms of a new player getting access to all the spells and resources upon joining, that would now be impossible because we no longer keep track of historical resources, and spells are all unique. If they joined they could get a copy of what other players have currently, but they aren't gaining anything that was historically granted that way.
It's not simply a matter of the old model was giving players a copy of everything -- it literally kept track of everything the players had centrally in one list, and then kept track of what each player had spent. And the spells weren't just handled globally in some stockpile, they were literally just flags saying "this is now unlocked" whereas now spells are actually unique objects unto themselves, with modifiers and so forth. A central list of flags wouldn't cut it.
For someone coming in late to a world, on a small private game, I don't see why the first player can't just give them a spellgem. It is not going to be long before the players are trying to get spells of the NEXT level anyhow, and so within an hour or so any sort of gap has long-since disappeared.
I know that's not the answer you were looking for, but I really can't understand the use cases where this would be a running problem with a group. If you're playing with a group that plays together regularly, there's no problem. If you're playing with someone who joins your existing game new, you just give them one of your spells and they are caught up quite quickly.
Especially with the more recent changes to the game, you now get resources AND spellgems themselves far more rapidly. You could literally have a new spell for the new player every 5-10 minutes if you go after a bunch of missions.
We are working on something big coming up for the game, though, and your comments are relevant for what we're doing on that. I'll definitely keep them in mind.