I disagree completely on this. Adding any custom enchants would make it just worse.
If you want custom enchants go and play normal factions which died due to them.
If any custom enchants would to be added it should only be as before(speed 1, strenght 1, haste 1/2, water breathing...). With this kind of enchants you could make a raiding set basically, and those enchants were mainly used for that. So nothing actually custom would get added, just potion effects on armor which give infinite effect lasting.
And also, doubt anyone would bother spending x amounts of exp on enchant books, mainly becouse all bigger factions would rather invest exp in spawner upgrades which makes them money. Even if they do make a "god" set with this enchants they would probably lose it the first time they wear it to pvp so again, no point in custom enchants.
EDIT: You said this could be a potion alternative for richer players, that would make the gap between donators and non donators even bigger(I'm not saying non donators can't be rich but if we look closely it is mainly donators having good stuff and exp to spend in this case).