I played a lot of forum system roleplays based on D&D3.5 and Pathfinder 1e, but not ERP ones, and with multiple players.
I tried DMing 5e ERP twice, and I have to say – I did not enjoy it. I find that Systems are useful when there is more than one character – it guards one from being too kind to players, also helps with dealing with natural human thing of having favourites.
But those things are quite useless when playing 1 on 1, some of the things I did not enjoy:
- You are not going to kill the character, the RP would end, sure you can kill them…but then they regenerate character, and have to start more or less from the beginning. When character dies in party of many – they rejoin them on quest, which is more natural.
- You are playing with single character, as DM you will have to be nice to the character and play it more or less easy-mode.
- Player does not have to compete with anybody, or cooperate with anyone, unless I create a henchman for them.