"Old English fyr "fire, a fire," from Proto-Germanic *fur-i- (source also of Old Saxon fiur, Old Frisian fiur, Old Norse fürr, Middle Dutch and Dutch vuur, Old High German fiur, German Feuer "fire"), from PIE *perjos, from root *pa?wr- "fire" (source also of Armenian hur "fire, torch," Czech pyr "hot ashes," Greek pyr, Umbrian pir, Sanskrit pu, Hittite pahhur "fire"). Current spelling is attested as early as 1200, but did not fully displace Middle English fier (preserved in fiery) until c. 1600.
PIE apparently had two roots for fire: *paewr- and *egni- (source of Latin ignis). The former was "inanimate," referring to fire as a substance, and the latter was "animate," referring to it as a living force (compare water (n.1))."
- from etymology online, a really excellent site and useful resource.
The pleasing idea of a process as an item comes, perhaps, from its already half-magical or numinous state. A process bound is already a little bit more interesting than an inanimate thing. We can see this in magic items, to show their magic; they live. The sword burns like a brand the jewel glows like a lamp. The cloak moves in an unfelt wind. The picture talks.
But even without that, a process bound, carried and sustained is slightly magical, it exemplifies power over an ever-changing nature, not just tool-use but bound process control. The common bindings break down quite neatly by classical element;
Fire is the big one. The key thing here is that it's not a magic lantern or a magic torch, but a magic fire. It is the fire you carry and preserve, not the means of its propagation (though you will need that as well).
The nature of the thing puts hard limits on how you deal with it or transport it. Fire is hard to carry and keep going, all kinds of common circumstances and opponent action could put it out. You need to keep finding fuel for it. Perhaps a particular kind of fuel. Magical fires could feed on human hair, the bark of certain trees, polished coals or fossils. It's easy to imagine someone carrying one, in a lantern, a brazier or something, but hard to imagine them carrying two or more in anything other than special circumstances. The nature of fire means that you will tend to adopt a ritualistic attitude to taking care of it, which embeds a principal ritual from the human lifeworld into the game. Because its very hard to deal with you can make it situationaly very powerful, PC's have to invest resources and through into how to sustain this powerful but delicate tool.
As well as it already being useful, it does so many things, there's a whole range of extra stuff you can do with it, shine its light on someone, breath in its smoke, let it's smoke 'write' on a piece of paper, burn things in it, cook things with it, forge things with its heat, let it cast shadows, burn yourself with it, see things in it.
Fire likes to get out of control though so this is a big difference to any other kind of 'item', and a D&D PC is going to want to let EVERYTHING BURN so then you have SUPER-POWERFUL fire. So some limitation of this has to be built into the thing. Maybe as a fire gets bigger and more powerful it becomes more intelligent, starts developing it's own ideas like a runaway AI or a bound daemon, starts using its powers for itself rather than for the PC's. And of course, few fires really want to stop burning.
Perhaps the fires are ancient daemons or angels. It makes a neat sense that primal demiurgic beings would be incarnated as natural processes. The idea of God making kinetic and other kinds of energy from, essentially, minced-up angels, seems legit. If you could catch a fragment of that Pure Fire from before they got all mixed up, and keep it going and preserve it, then you might have a little piece of divine magic that did a particular thing.
Is there anything that relates to moving water the same way? I mean not just that it interacts with water but that it needs water to activate it, to make it live.
A mill-wheel might work, difficult to consider carrying it about, though the idea of an adventurer or NPC with a giant fucking wheel on their back is interesting, mendicant mill-monks.
A portable water-clock seems like an edge-case, too mechanical.
Rain, the water that drips from an umbrella having special qualities. It would have to be natural rain, not just stuff you poured on there because it is the process that brings it alive. Maybe it would tell you things about the rain, perhaps heal you of mental disorders, curses or shame or simply help you forget.
Those lantern-boats that float downstream might also work, carrying things away or summoning them back. Water ghosts, creatures from the past or those lost.
I think it was Keats (or maybe Shelly?) who's epitaph was that his name was writ in water. Perhaps writing someone’s name in water causes someone, somewhere in the world to forget that name. You can perform mass-attacks on someone’s fame and reputation by gathering hundreds of cultists by the shores of a still lake and having them all write the same name on its surface. The name attacks must inevitably affect the cultists as well as everyone else so every now and then some of them must forget the name they are writing and have to look over at the guy on the left and right to see what name they are erasing. If this carries on for long enough, everyone in the world might forget someone's name except for the people writing it on the surface of the lake, and even they don't really remember why they are writing it, only that they were really pissed off with whoever it was.
A Chinese Whispers effect could mean that even people with a similar name suffer some effects of the attack.
A brush that, if used to write on a waterfall or the surface of a river, or even on a wave in the ocean, you can summon or control the power of those things. Perhaps the symbol for 'Horse' on a waterfall creates a charging steed of white foam that can only race downhill and which ends each ride by diving and exploding into the river or the earth, writing it on a river creates a tireless horse, but one which can only ever move at the speed of the river itself, whether fast or slow and writing one on a wave creates a titanic and powerful horse that can charge along the coastline, but not beyond it, plus getting in position to write on a wave as it breaks effectively makes you a surfing wizard?
Between water and air we have the sailing ship. Perhaps a sail that when it runs directly before a headwind and pulls a ship to full speed, can breach the barriers between planes, like a sailing DeLorean. The precise direction of the wind affects which plane you go to and you have no way to control it other than to have it up or not.
This brings us into air. Kites are probably the closest equivalent to torches and lanterns. Maybe a kite that, once you get it flying, transforms you into a bird until you touch the ground. Perhaps if you land on water you can stay as a bird until you reach the shore so people passing through never know if the ducks and swans are kite-monks in disguise or what.
Pin-Wheels. Even breath could activate them but they are incredibly delicate and easily damaged and destroyed, hard to cart around for a long time without them being crushed, so that provides a nice limiting element. Perhaps they have to be activated by natural wind rather than breath, the idea of a brawny adventurer running about with a pin-wheel is a nice one. Perhaps the breath of certain creatures or type of person is needed to activate the rare ones, a virgin, a holy person, a seventh son, a blind man.
There is the lightning-charged device of course. We would have to work on ways to make it more interesting than just a lightning capacitor. Perhaps, at the moment you catch the lighting you gain the speed *of* lightning, you can race anywhere but as soon as you stop the lighting is grounded. You also have to be careful not to interact with anything touching the ground, you may be drawn towards metal objects and end up accidentally hurling yourself on a drawn sword 300 miles away at the speed of sound.
Monks who wait under trees in autumn with special brushes. They train until they are able to write upon a falling leaf before it hits the ground, touching it only with the tip of the brush. That would require huge study and skill to use. Perhaps you could use this to travel through time like a leaf falling.
Stone is a process too, but one so slow that I don’t think it could be contained into a pseudo-object by human beings.