- ban grinders
Nah, let people make grinders. As long as they're careful (not walking around paying for things with a ridiculous amount of heads), nobody should suspect they have one, and it causes no problems. If they're not careful, fights would break out over the grinder and it'll probably end up disabled, if not destroyed. Conflict weeeeee
But uh, the permission to utterly destroy any grinders you find should be added to the rules just so that people can't ♥♥♥♥♥ when their OP XP farms are ruined.
And really, all you'd need to do as shopkeeper would be stop accepting skeleton/zombie heads. Creeper would still be cool because no spawners for them.
Addressing your other stuff:
- new members forced to join guild with least members if all the guilds are not evenly numbered
Sure, as long as there's no settling restrictions. I am vehemently against it if it means settling in a biome I do not care for, and I think many of the more aesthetic builders would be with me on this.
- maybe (?) random guild members (excluding leader who is not random)
Yes please, it prevents "power players" from sticking together (hopefully) as well as encouraging interaction with other factions.
- some sort of economy
Taken care of, using heads as different denominations of currency (will require head plugins).
- something to enforce more rp
Pretty much covered with the above points, although tweaks could be made after seeing how well things pan out.