If you wanted to go about the End the easy way, just have a crafting recipe that results in an End Portal Block. Just a suggestion. Great work either way!

I think if you wanted to go about this the easy way, just create a minecraft server with Forge, the Morph mod, a cat mod, Mo' Creatures, and custom mob drops, and two plugins, ranks and factions

So when you start out by logging into the server, your name is (username)-kit. You start in a room with a bunch of cats in it, you can choose your appearance by killing one of the cats, after you choose your cat you could walk out of the room and random TP somewhere.

That's the first part.

You could join a Clan(represented by factions) or become a Rogue and fend for yourself. Individual players could create clans or you could have six main clans with no ability to make more.

Im not sure where you would take it from there, but it's a start.

I think your idea would be pretty interesting and add a twist to the game. I would love to work on it, and I'm pretty sure it wouldn't be overly challenging. I have a few questions though.

Would you like them to spawn in the same general area as you?

Should they respawn(Obviously not in hardcore)?

I think adding some RPG elements like stealing would help your interactions with the NPC's, any other ideas?

I think I would also like to add factions, or clans, so you can team up with the NPC's. Any objections?

It's a really interesting idea for a mod, thanks for giving me something to work on over the summer!