One of the key elements of making Eberron a high magic world is the way in which the Gnomes of Zilargo are able to bind various different kinds of elementals (Earth, Air, Water, or Fire) into different machines. Their inventions include the lightning rail lines, airships, tunneling spheres, and elemental enhanced waterborne ships. All of these involve calling, as opposed to summoning, an elemental, as the former moves the elemental into the realm of Eberron rather than a kind of immortal projection that summoning produces; an elemental is under no risk if it is summoned.
In “The Magic of Eberron”, there is a group of Gnomes who support an idea of negotiating with elementals rather than just ripping them from their plane and forcing them into servitude. Unfortunately, they don’t go into any detail as to how such elemental machines would differ from the default, or what the caller would have to do differently. They even suggest that various new kinds of enhancements for both equipment and the bodies of casters, that would require extracting parts of elementals. There is no mention on whether or not these extracts are lethal or something the elementals can heal from; one would assume that since these new enhancements are couched in the realm of the negotiating Gnomes, that it would be a non-lethal donation. Once this technology is discovered, however, it would be reasonable to consider elementals being called against their will and harvested for their ‘organs’ by less charitable Gnomes.
Overall, I believe that the elemental binding should be considered, evaluated, and adjusted in regards to several different factors: willingness of the elemental, duration of service, selection process, and compensation.
The default condition presented in Eberron I believe to involve unwilling elementals, bound for an infinite duration, randomly selected from all available elementals on that plane, and having no compensation for their work.
The first factor under consideration is willingness. If the elemental is called and successfully negotiated with, the elemental should be considered willing to partake in the binding. This negotiation might involve promises of freedom after a set period of time; the elemental is willing to partake in this experience because it will get to experience a world it would otherwise not visit, knowing that it can return to its home plane after a set period of time. A subset of elementals might enjoy the process of binding, finding the experience to be pleasurable, and so finding that population would be of great use to binders. Finally the elemental might be given compensation of some sort, either as materials, such as gems for Earth elementals, enchanted water for Water elementals, etc. that a price tag can be placed on. Or perhaps an exchange of services, so that the binder would have to perform some task.
A willing elemental should be easier to control, perhaps not even requiring any control rolls so long as the elemental recognizes the operator as legitimate. There would be little chance for the elemental to go berserk, and even if the binding is damaged, since the elemental was called, it might remain to assist the operators of the vessel. Imagine a scene in which a Fire elemental, part of a now crashing airship, scoops up a child, and while burning them, deposits them safely on the ground. These elementals would be partners and would resent the use of various spells or devices that injure or inflict pain upon elementals to ensure their compliance; they might be harder to usurp control.
The duration of service isn’t just a matter of the total length of service expected by the elemental, but potentially their service over the course of a day. There aren’t any notes as to whether or not elementals tire or expend energy to perform the task they are asked of, but they may bore from containment, and may ask for relief from time to time. This might mean that there shifts of elementals that operate a particular vessel, with some kind of more comfortable storage facility that they can entertain themselves between shifts; this would require more than one elemental bound per vessel. Further there is the question of how long can an dragonshard contain an elemental; most containments in Dungeons and Dragons work by the principle of containment until the vessel is broken, so a long contained elemental might be insane.
The selection process might be looking for the same elemental that previously served a stint with that particular company or on that particular vessel. Some elementals might, as mentioned before, enjoy the process or the experience, but might also be on the run from authorities on their own plane; better to serve, called, in Eberron, than be punished or even killed back home. There might even be a kind of slave trade in elementals that parallels our own planet’s slave trade, where powerful elementals might round up weaker elementals and trade them to binders in Eberron. Certain elementals might be bred or raised to have a slave mindset, or might be criminals. These could result in either lower or much higher chances of trouble with the elemental onboard, and if they’re wanted criminals, elementals or their agents might try to track them down. On the extreme other end, the selection could be entirely random, so elementals on that plane have no warning or idea who might be plucked from their plane next.
Finally when one considers compensation, there is, as has been mentioned, mere gold exchanges (or their equivalent in gems, incense, etc.), trading favours, but it might also mean modifications to the circumstances of the binding. As mentioned, they might want to have a special enclosure to relax in, something that they may not have access to back on their own plane; a kind of gilded cage. They may ask to be released into Eberron rather than returned to their plane, or they might ask to be returned to a specific part of their plane that was far distant to where they were plucked from. They might work so that other elementals bound would be made free, or ask that a kind of body, like a modified warforged, be constructed so that they may blend in more easily.
Generally speaking, if the binding ‘costs’ the binder more, it should result in bonuses; increased speed, maneuverability, control, reduced chance of rebelling, etc. This all assumes the binding of default elementals. There are ‘celestial’ and ‘abyssal’ types, such as celestial dogs or abyssal spiders, so why not elemental horses or other beasts of burden? The resultant binding may not be as powerful, or may be more powerful but lack control, as the elemental isn’t as smart when it comes to how orders are relayed. When it comes to using elementals, a lot more consideration can be taken to give their inclusion in Eberron greater depth.