Real life mod; Ranching, cattle driving and rustling. Being an outlaw and have a posse in SP (using ped models instead of the protagonists). Be able to sell cattle at the livestock towns. Be able to work on the ranch, buy some of the ranches available in the game and work on it.
Breeding horses and selling them aswell, since you can deliver a foal in the Epilogue, that might aswell be available for modding or simply giving something similar to be worked around on a mod. Be able to capture wild horses, get them back to your ranch and take care of it, feed it and then sell them in the market, make some profit.
Be able to use a ped without losing dead eye and eagle eye for hunting. I once tried using the native americans ped and I unfortunetly lost the ability to eagle eye and dead eye for hunting, it was a pain but free aim got me through with it.
Be able to spawn any horse in the game, max out bonding level with a click and be done with it.
Be able to grow hair and beard with a click and be done with it.
Brothels.
Wifes.
Really, Real Life Mod from GTA 5 would be amazing in RDR2, I guess this will become real when RedM arrives, but would be awesome to do all this in SP.