For me the rarer the better, if you add something that everyone can just get then in my mind there’s no reason to add it at all. I for one LOVE having mounts and items people will never be able to obtain again and they are some of the only things I use as mounts and transmogs because you rarely see them.
I would prefer the ladder. 100% guaranteed mount but for doing something hard or getting a achievment. Like mythic+/Raid/PvP mounts.