1. Warships, sailboats, wagons, carriages, horse-drawn carts, huts, houses, towers, castles…all of these can be magic items, and it follows they can be attuned to.
2. I think a magical property that magic conveyances and magic structures could posses is something called Shared Attunement: The owner of this magical structure/conveyance may share the magical properties of any other magic item to which the owner is attuned to with the structure or conveyance. This is accomplished by placing the item on the structure/conveyance (hanging a magical shield on a bracket on a wall), and spending a short rest linking the attuned item to the c/s. Once this is done, the c/s is considered to have all the magical properties of the magic item, and the owner of the c/s may use the shared properties from anywhere in the structure, or while in physical contact with the conveyance.
…a DM would have to decide odd cases or corner cases, but I think this basic idea is workable.
2. So, let’s do that. If a player builds a castle, gives it Shared Attunement and decides to hang his or her trusty Flame Tongue on a wall in the sitting room, what can happen next? Well, according to your description, the PC should be able to speak the sword’s command word and cause the castle to erupt in flames that shed bright light in a 40-foot radius (really this ought to be farther–I mean, we’re talking a freaking castle here). Presumably the interior rooms wouldn’t light on fire, and the castle exterior would remain as immune to the flames as the blade of the Flame Tongue.
3. What if the sword was a Luck Blade? Hrm…well, the PC wouldn’t gain a +1 bonus to attack and damage rolls unless he or she was actually wielding the blade. They should benefit from the +1 to saving throws, since they are in their castle, which is about the same as the sword being “on their person.” That means the PC would be able to activate the Luck and Wish magical properties of the sword from anywhere in the castle.
4. Forget swords. What about Staves? Specifically, the mother of all staves: Staff of the Magi. As with the Luck Blade, the owner would not gain the +2 benefit to attack and damage rolls because they’re not wielding the staff; it’s hung up somewhere in the castle, in a (presumably) safe location. The owner would gain the +2 to to spell attack rolls, since being within the area of the castle (and in contact with it) is similar to holding the staff. Therefore, all the other magical properties of the staff that require it to be held in order to be used (Spell Absorption, Spells, Retributive Strike) can be used from anywhere in the castle.
5. [Encounter Idea]Whoa…that’s pretty powerful. But if we switch this scenario from a PC to an NPC, then you have the makings of an interesting ongoing set of encounters in the dark castle of a Big Bad Evil Guy NPC, who just happens to have one or two secret rooms in which their Staff of the Magi and another powerful magic item are kept, so the bad guy NPC can keep their hands free to do other nefarious things to the PCs while being able to call on those two items’ powers. If they PCs can get to the items first, though, they can limit the bad guy’s power by destroying the items or maybe turning them to their own use somehow. The PCs could also figure out how to levitate the NPC so he’s not in physical contact with the structure, since this would be like taking a magic item out of his hands.
6. I figure a Retributive Strike called up by the bad guy NPC in the scenario above would not destroy the Staff, it would explode the whole freaking castle. <insert evil DM laugh here> The description does say you have to break the staff, but I suppose the rules could be bent if the NPC could break something else attached to the castle (like a window).
7. Hmm…what if we power everything down a bit…maybe an item, like a wrought iron decorative hook–like a coat hook or something you’d hang your walking cane from–that can be placed on a conveyance or fastened to a wall in a room inside a larger structure. The owner of the magical iron hook would have to be attuned to it, and any magic item hung from the hook would be considered as still in contact with its owner. This way, a sleeping NPC (or PC) who doesn’t want to have to lug their Staff of Power with them to the jakes in the middle of the night would at least have a shot at using the powers of the staff to defend themselves if they were caught unawares in the structure. An NPC riding in a wagon would probably be using both hands to hold the reigns controlling the draft animal(s) pulling the wagon, so this item would allow the NPC to fight back without having to let go of the reigns and grab their magic item(s) up.
8. Another Minor Property: Faraway This item never becomes unattuned to its bearer as a result of being separated by a distance of 100′ or more over a 24 hour period. This property does not extend beyond planar boundaries; the bearer and the item must be on the same plane for this property to function.
9. There ought to be objects or compounds or alchemical mixtures or spells or magic items that modify the process of attunement during a short rest, for better or worse. This is a topic for another day.
10. Regarding Attunement in general: Maybe some items that must be attuned to be used, and that remain with one owner for decades or even centuries (think: Liches and Dragons) end up becoming tethered to their owners in ways mysterious. For example, if a PC finds a magic item that requires attunement, and then discovers that they can’t attune to it, then the spellcaster in the group might be able to deduce that this means the ghost of the last owner of the item is still on the Prime Material Plane, and that former owner needs to be put to rest permanently.
11. This could be a simple “slay the evil ghost” adventure, or something more involved where the PCs have to figure out what wrong was never righted before the former owner died, or something similar. Succeeding in this way might grant the PCs a boon of other magic items tied to the ghost, that are now free to be attuned to the PCs.