illyisabot
In my personal opinion both are great ideas, for the first one he can rely on the existing harry potter work for much of characters, he will also have an already established plot, to which he can make changes and hence changing the plot, he will have a storyline to rely on, but for the second idea he has to be lot of innovative and should create lot of plot on his own to make it intresting. Both have pros and cons in the first one people mostly expect their fav characters to be same or have a better life hence may fell dissapointed if their aspects and behaviour are changed, but for the swcond idea he can explore and innovate his own ideas and can go all out as there would be no pre established expectations on the characters, knowing these he can make his choice, but before doing so stick to the story to which you can envision a proper ending.
Both ideas are good, if you choose the first, there is the opportunity to use Harry Potter and Legado Madilto. Honestly, it has a lot of potential. In addition, the truth is that I have never come across a fanfic that deals with this. In addition, if you didn't know it, it is canonically acceptable since J.K.Rowling already said, I think that once this would be the last book in the saga, you could also use this part of the saga as a base, there your friend can go and innovate if he wants. And the second option, if it is in the time of the founders, the truth is that I find it very complicated and difficult, more than anything because you will have to be very creative and create a solid plot. It would also have to not be to have the objectives of being immortal or maybe if you join The founders say that creating a lasting legacy is understandable where I am going with this. I am just saying that if you choose the second option you will have to be very creative but the good thing is that you will have a lot of room to innovate and more things like that you have several options you could even enter into historical wars the truth. Pd:recommendation not to try to be more than one fanfic or else everything will be complicated, no luck to your author friend
If it's during the founders' era, then wouldn't the Protagonist die of old age before the events of canon? Or would he get something similar to eternal youth or immortality to last that long? Or maybe being a ghost he investigates ways to cast magic or strengthen his ghostly soul like a shinigami from bleach? Maybe even investigating, like in bleach, the creation of an artificial human body capable of being used by souls to interact physically on the plane of the living.