As someone currently working on a plug-in I'll offer some advice from my knowledge of how POG plug-in making is designed with Coldstone.
First do not attempt to completely re-write POG with a plug-in. You will end up in a
mental ward actually believing that you are a squirrel. Trust me on this one.
Actually thats the only advice, heh, but I will expand upon it.
Coldstone will not allow you to remove current NPCs, shops, etc. Nor will it allow you
to easily change the lay of the land(although this is possible). You can not make a
simple hack to change a "Utian cap" into a "Shire cap" for instance. It is very very
difficult to change the existing POG world, and to be frank about it, I personally do
not think the time consumption it would take to physically alter the world to be worth
the payoff in the end.
Now, that being said, I do think a world-altering plug is not only possible, but fairly
easy to do. What's that you say? Did I just contradict myself? Heh, probably.
What I would do if I were going to write a LotR plug-in or any other type of genre
altering plug-in, I would create all new maps, new items, new npcs, etc. Then I would
create a global and initialize it to 0. I'd then hack the starting sequence and make
a call that if the global we created is equal to 0 then teleport the player to the
starting location of the new map you created(wherever that may be) and then set the
global to 1(so that everytime the player logs in he isn't teleported back to the starting
area!). You would also need to make a check in the load game system call to see if the
character being loaded was saved in your plug-in(ala. the way trinity says, "This player
was created without Trinity."), and in the instance of a completely new world I would
suggest if this is the case that you do not allow the game to load(effectively requiring
the player to start a new character in your new world).
You can then proceed along with the plug-in.
Doing this would eliminate the need to re-write all of POG; everything would be new. You
also wouldn't need to do the kludge of having something in the game that will teleport
you to the "new world." Ie, there would be no cross-worlding allowed.
These are my thoughts on this type of thing and my advice. I haven't attempted the
above at all, but looking at how Trinity works all of the above should be possible for
the creative mind.
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"In the mornings, when I'm usually wide awake, I love to take a walk through the gardens and down by the lake, where I often see a duck and a drake, and I wonder as I walk by, just what they'd say if they could speak although I know that's an absurd thought."
-- Conjunction Junction by Schoolhouse Rock