Options for human head transplantation

There are several options:
(1) Connect head to a living person. Here we find a living person who wants to have another head be connected to own body.
List of possible reasons to agree to be head transplantation recipient
(2) Connect head to brain-dead body.

(1) Connecting head to a living person

Pros: (a) the recipient person can fullfill some motoric wishes of transplanted head to some extent like going somewhere or doing smth (that can be part of the contract, the easiest part to be sure).

Cons: (a) recipient would receive severe immunosuppression that would negatively influence health. Moreover even that severe immunosuppression might turn out to be not severe enough (compare: "The donor body's bone marrow, spleen, and even lymph nodes can be sub-lethally irradiated with appropriate shielding for all the vital organs, including the thymus"<ref>https://pubmed.ncbi.nlm.nih.gov/28130190/</ref>).
(b) this approach is not easily scalable. There are not so many people willing to supply the head of another person with own blood, especially considering possible negative consequences for their own health. While very wealthy people could motivate recipients with money, most of people can't do that.

Options:
(1.1) The head can be surgically sewn to the new body.
(1.2) The head can be connected to new body via long vessel cables.
Pros of (1.2):
(a) the surgery is much less traumatic
(b) the surgery is much easier and cheaper
(c) in many aspects it's more convinient for both people. They could even probably live in separate rooms (with a cable connecting them through small sealed opening in the wall).
(d) as blood going through the cable can be potentially monitored and controlled, there potentially might open opportunities for better controlling possible immune rejection reactions? (at least acute episodes?) by possibly some form of clearing the blood from immune cells?
(e) potentially even two/several people could supply blood for head donor, f.e. exchanging each other. So it potentially could be like normal work, with like 40 hours a week.
Further options for (1.2): (1.2.2) head is not separated from old body except brain arteries and veins are disconnected from old body and connected to new body through long vessel cables. As long as patient body is functioning, patient would be motorically and sensory fully functional (though why the patient body would be functioning if the whole procedure is performed because the patient body is near the dying edge?) It might have sense if the patient's body is not near dying age and the whole procedure is performed just to try to rejuvenate brain... which doesn't sound as a good idea nowadays though in future that might change?

(2) Connecting head to brain-dead body

Cons:
(a) This is hardly scalable approach. There are not so many people dying for brain-related reasons and only small part of them wishes to donate their body to new head. Also, for every new head saved due to transplantation to brain-dead person, there must be perhaps a hundred of died people (of which one died for brain-related reasons and wishes to donate the body), so only a very small fraction of people could save their heads with such an option.
(b) It's bad from ethical committees viewpoint because brain-dead body can be source of many organs for transplantation which can save several lives instead of one[1] (and each of those several lives are expectedly longer and of better life quality).

Options:
Old (brain-dead) head can be preserved (2.1) fully, (2.2) partially, or (2.3) may be replaced with new head.
Cons of (2.3): Complete spinal injury raises hard problems. The head would not be able to breathe normally and would need ventilation machine. For example, person after C2 spinal injury can not breathe[2]. The brain respiratory center would still work as expected and give commands to diaphragm routed nerves so if we sew those nerves with body diaphragm nerves or make some system that detects currents in head outgoing respiratory nerves and stimulates correspondingly body respiratory nerves... or detects oxygen level in blood and stimulates diaphragm... but if it all worked better than ventilation machines it would already be used instead of them. There would be life-threatening problems with blood pressure regulation as well[3] and other cardiac disfunctions[4]. There would be no way to cough or sneeze timely which is important for clearing mucous from the lungs and failure to do so leads to infections like pneumonia[5]. Anyway life in scenario (2) is strongly tied to a hospital bed, constant 24/7 monitoring with many devices and to intravenous drips.
So currently bicephalic option is better because of the old head (or at least its brainstem) playing important role in body respiration, blood pressure regulation, etc, as well as because of less traumaticity of surgery. In the future, when spinal cords could be fused, replacing head might be more convinient. Anyway this method doesn't really scale well into the future. Though it's hard to say which of (2.1)-(2.3) options is better from ethical committees viewpoint. F.e. they might choose (2.3) to prevent any negative qualias in the brain-dead brain, thus we could face quite massive cons of (2.3).

Possible future options

(3) Connecting head to artificial body

Currently, there are devices which can fullfill function of one or several organs for some time, see Artificial organs.
To the best of my knowledge, I would argue that there is no combination of devices that could support life for considerable time in the case of multi-organ failure.
However, if (1) we need to support just head and not the whole body, (2) we can buy new donor blood from time to time (may be even every day) then perhaps they would success? Needs further analysis.

Pros: more easily scalable than (1) and (2)
Cons:
(a) probably requires more time to be developed
(b) While artificial multi-organ support system is of great importance (and seems to have decent funding), will we really need to isolate head if we have a system which can support the whole body? see Can head transplantation to artificial body be the best option?

(4) Connecting head to animal body

In some future there would arise two other possibilities:
(4.1) Animal body bicephalic (human head + human body + animal body)
(4.2) Animal monocephalic (human head + animal body)
Probably (4.1) is better than (4.2) because animal head would control basic respiration, blood level control etc.

(4) is somewhat harder than (1). Despite shortage of organs, the first transplantation of animal heart to person happened just several months ago. There is a person who lived for two months after transplanting the heart of a genetically modified pig. This likely gives us an estimate that human head/brain/upper body probably can be transferred to a pig though there is a reason to think that patient probably will not live much longer than several months after operation.
However, rejection of pig heart is very different from rejection of brain by pig blood immune cells.
The pig might be severely immunosuppressed and be kept in sterile conditions and on antibiotics (though transplanted head would have those antibiotics as well). If pig gets ill, the transplanted head could be switched to another pig while the first pig is being treated.

Pros of (4): In some future the option (4) potentially can decrease surgery price because human recipients willing to be sewn second head will always be in shortage and expensive. Cons of (4): Blood composition mismatch (though not that large! actually they are similar in many important ways[6]). Some pig hormones influencing head in unknown ways and in known ways (if pig gets angry you probably would get angry too?)

We could grow genetically modified or chimeric animal with low immune response to a given person.

(5) Connecting head to artificially grown anencephal clone

Another possible option is to grow anencephalic clone and transplant the head to it. This option has really good as well as really challenging sides.
The really challenging sides are:
(1) ethical committees
(2) most of anencephals currently don't live past several days/months with one of the records of 28 months (though that might be largerly due to common neglection towards them; nobody really tries to prolong their life). However anencephals who live longer tend to have more intact brain stem (and maybe even hints to some other structures). The more brain structures they have the more problems potentially ethical committees would see.
(3) still as for now it needs a woman who will bear that anencephal clone and that would not be too cheap; so preferably technology for growing the clone in animal or in vitro should be developed for the option to be scalable
(4) specific problems like bedsores etc.
The really good side is, it's scalable once the challenges are solved.
Importantly, anencephal clones could potentially be useful for organ transplantation as well. Organs are always lacking, so if there would be some real possibility to make research in this direction, perhaps more mature field of organ transplantation would do that.

Options for even more distant future

We could just grow new body in vitro (it would lack head and brain from the very beginning so no ethical problems) perhaps with a great help from deep learning. Alternatively, we could grow organs one by one and then connect them. All of that could have the same DNA as a person.

Misc: One of the best available scenarios is identical twin who agrees to have a second head sewn on from a twin (identical twin translantation requires much less immunosuppression). However identical twins are relatively rare.