How about a setting for a World that would allow two people to play the same account concurrently? I think it could really be a help for people just starting to play who are being mentored by more experienced players.
It would be handy for that purpose actually. Though I imagine people would complain because two coplayers who knew what they were doing could play at the same time as well though I honestly don't see much harm in that either.
The one bonus this has compared to some other things is that sidding (what this basically is) is easily detectable, and compared to other rules, more easily enforcible.