Hey Jenova11.
Although I tend not to post much here (I tend to be both A) busy and B) an amateur in a lot of ways compared to these guys) I can offer you some answers (although not all of them) from my own research and attempts to modify a TC server to my liking.
I primarily speak English, and even then I'm not particularly good at understanding things if they're not spelled out to the letter, so if I end up answering something you didn't ask about I'm probably misunderstanding you. ^.^;;;
Question: Is it possible to radically change the gameplay in world of warcraft?
Answer: Almost certainly. All of the gameplay is controlled by the server with some assistance from the client. Both the server and the client need the same information in their shared files, but it's doable.
I'm currently working on getting my server to handle the player's primary and secondary stats very differently, and so far there's been no real problems (except when I mess up the code.
)
Question: Add one-fourth talent tree
Answer: Although I haven't done it myself, (I haven't gotten there yet) I know it's entirely possible to alter, remove, or even add talent tress. I actually read a how-to on this somewhere. I think I bookmarked it, so I'll start going through the bookmarks and trying to see if I can't turn it up.
Question: -Radically changing the allocation system such talents as the game with allods ruby.
Example ruby on the game allods online.
Each lvl in addition to winning a talent point saves ruby to validate a checkbox.
Answer: You mean something like... Confirm that they want to spend a talent point there? There's already a system for that in place in WotLK, (3.3.5a) and although it seems a little glitchy under Trinity it seems like it could modified to server that purpose.
Question: And more generally through a client development Lua / XML interface for server and core (mangos) can add your new system?
Answer: I can't speak directly for MANgOS or however it's written, as I use Trinity. But Trinity forked from Mangos, so I expect that the answer's likely the same.
Trinity only currently has a prototype LUA scripting engine for the server, but you should be able to use LUA/XML to modify the client GUI as it's built on that. In fact, this is what Blizzard does: From what I've been able to figure out, most of the 'changes' to the Blizzard GUI are actually only 'Add-ons' that the client installs and automatically uses. You'd probably need to learn how to work with Add-ons, but with that knowledge you could alter the GUI.
If you wanted to make changes to the core descriptions and things (IE: races and classes information at the character select screen) that's trickery. The information is still in XML/LUA format, but it's stored within .MPQs in your data<region> folder (IE: for me it's dataenUS). That's a little trickier to modify as it does seem to be validated - or possibly checked - by the client or the server in some way. But I've seen others bypass it, so I know you can change it. I'm just not sure how.
Not-Really-A-Question: If the answer is positive the possibility deviene truly infinite.
-Game Store
Novel mechanism-
New location-equipment ...
Answer:
At least with Trinity Core, you can write additional scripting to handle all sorts of things, and I've seen someone host a server before that allowed you to switch between resource mechanics, so I know all kinds of things are possible.
New locations and equipment aren't exactly easy, but the people on this board are experts about them, and you can see a lot of the awesome work they've done along with a few guides on various facets of it.
I'm not entirely sure what you meant by 'game store', but the most likely answer is that's possible to.
I hope this helps reassure you about what the actual limits of WoW-modding might be, as long as you're willing to put forth the time to actually effect the changes; and maybe even helps answer some of your questions.
Cheers,
-Rimewynd
