So here is what I am thinking...and mind you, this could change by time we play on Monday night (or it could change by time I am writing the post)
Provisions of House Rule
All magic items require attunement. The two general exceptions are alchemy items (potions, powders, oils, bubble bath, body lotion, etc) and scrolls. Many gnome devices, clockwork or otherwise, do not require attunement. The finder's lens (purchased by the player characters in Session 9, it is actually an alchemy item) does not require attunement, nor does the transportation vehicle control amulet (found in Session 41, attunement is instant and automatic). However, the Chalice of Doluth (creates healing potions) does require attunement.
There is no limit to the number of items to which a character can be attuned.
Some items, particularly intelligent items, may have the power to unattune (detune?) other items in the character's possession (cue evil DM laugh). This would suck, but it makes it interesting for the DM. This is what matters to me.
Some items may only be attuned by a particular class or race. Or there may be limitations placed on its use when a particular class is using it.
Attunement to one item can be done during one short or one long rest. A few items may be created with the feature that it instantly attunes on contact or another condition.
Most items require a dice roll for attunement. The Difficulty Class for low power common items is 10 (easy). Characters add their Charisma modifier, plus their proficiency bonus, to the die roll. If the die roll succeeds, the character is attuned to the item. If the die roll fails, then the attunement fails...and, more importantly, the character can never attune to that item (short of a wish spell, divine intervention or DM whim).
Limitations on Magic Item Use
We will follow the AD&D 1e rules regarding limitations on number of items used at any one time: 2 rings, 1 set of armor, 1 magic cloak, 1 pair of boots, 1 pair of gloves, etc. This is not part of the attunement rule per se, but it is related to it as it applies to magic item use (thanks to Peter D for the reminder on this set of AD&D 1e guidelines).
Implementation of House Rule
We will assume that the player characters have successfully attuned to all of the items that they currently possess.
We won't worry about the current bags of holding and portable holes for now. I don't want to waste precious game session time on figuring this out. We will just hand wave it for now.
I can think of two items, the Chalice of Doluth and the Lens of Melnar, that we need to identify an owner for attunement.
