So idle logout is going to be a concern here.

In most games, idle logout isn't a big issue -- you can always go back to where you were.

Here, you can't. Your server state is thrown out. State that took you time and potential money to get to.

What can be said about a client going idle? Well, one thing that can be said is that if all the players on a level are idle, the server process / thread handling that can go idle until someone wakes up. It can do a blocking wait on input messages from the client(s).

Being a blocking wait, it doesn't need to consume any server resources, including CPU; with no memory accesses, it can be paged out. Result: It doesn't have to consume any resources or disrupt any other process.

You might still think that this is excessive. Fine; memory consumption might still be an issue. So you dump the state of the level and the players -- their pickups, health states, etc -- to the database in a temporary holding table. One dungeon level -> one row and a to-many for the state of each player on that level. When the account logs back in, either present a "Restore last level, or restart in town?" dialog. OR, restart the level, and let them continue or leave party. Note that this second setup has the benefit of allowing restores after a system patch -- before shutdown, save all active levels/parties to the database; shutdown/restart with patch, and then restore from the database.

This could be extended to give an explicit "save game" feature. Useful if someone doesn't have as much time after all as they thought.

Now, a saved level state might be tossed out after 2 days. How long it's saved isn't the issue. Losing progress when you are 90% of the way through a game with no "save game" feature, where you can't even go potty if its going to be a big dump, where you have to stop and deal with your cats and dogs not getting along and needing someone to remind them that they are now a family, etc. Heck, even a parent that all of a sudden needs help from you with her medical issues.

Every other persistent world game I've played, from WoW to YPP, has had some way for you to rejoin where you were. Granted, YPP would lose the state of whatever minigame you were playing (grumble Alchemy grumble), and WoW you might wake up dead, but other than that, you could continue. (And neither WoW nor VO had any penalty for going idle in an inactive area, and YPP's worst penalty was the ship reporting). Here, you go idle, and you may have lost 50-70 energy. You can't just log back on and continue (saved state gone), nor can you restart (no more energy), so ...