I would say just add some type of check box when you pick the character you wish to play. Just like how you pick if you want to be Seasonal, Hardcore, ETC. One of the boxes could be “Personal Loot” now if you pick this option you can’t play with people or even see other games that don’t have this option turned on.
This option would split the player base but I don’t think it would make a huge impact. Mainly because the player base for D2 right now is small and the trade economy is pretty good and if all of them come over to D2R with the added people coming back to D2R that won’t turn on the Personal Loot option the economy is going to be pretty amazing with everyone playing.
And for the people that do turn it on get to have their fun playing with the Personal Loot. The economy would be different though.
I personally would have the option turned on. I remember playing D2 as a kid an I love the game but I had a miserable time while playing in groups because I never got anything. I did play solo at times to get gear but its just lonely at times.
Even without personal loot I will play mainly because to me and I feel for a majority of the now Diablo player base D2R is a temporary game until Diablo IV.