The one I would most like to be able to write is, unfortunately, a story that would take years of work and research to do right... the concept is that a couple from the distant future decide to go back in time and live the simple life in 15th century england- perhaps they are fugitives from the law in their own time... Maybe they are time travelling historians that broke a serious rule in their travels...

In their native time, disease and aging are unknown, nano tech has eliminated disease, and the deterioration of the body with age has been conquered- it isnt immortality, they are just as vulnerable to an arrow through the heart as anyone, but they dont get the flu or infections, they heal from injury far faster than normal, they can survive trauma that would kill most people.

They bring gold with them and are able to live quite well as almost nobility- They eventually have a child. They didnt plan on having a child, they just wanted to live out their long lives and hide from the authorities of their own time- but having a child is a tragedy- since the child hasnt been given the nanites and genetic drugs that they have, they will almost certainly outlive the child- in fact they will likely watch him grow old and die while they dont look a day over 30...

The child gets sick - they decide to take the risk of going back to their time to get the treatment that will cure him and give him the same longevity they have- they are successful, and he is saved, but their trip back the future has tipped off the 'Time police' They are captured and returned to their time- the child escapes and is forced to live as a near immortal in the past... The book would essentially be his diary, starting at maybe the age of 60 when he realizes that he isnt aging like he should- the story would document his life from the 15th century until the present day- the wars and famines, the steady change....

The hard part of writing it is that, to be believeable, the writing would have to reflect the writing of the time in which it was supposedly written, and the tone and style would have to change as the fictional character ages and changes... It would take a team of historians and linguists to do it right.

There is also the psychological aspect- If you realize that you will never age or get sick, wouldnt that change the way you live your life? If the only thing that can cause your death is some sort of traumatic injury, would you become a timid person that avoided all risk, or would you become an overconfident daredevil?