Yes, all these things are possible, but they are incredibly difficult to do. Very few people, as far as I can tell, have the knowledge to do these types of things, and don't seem very willing to share that know-how. I must admit, I, too, am one of those people.
I spent over two weeks, at about 8-10 hours a day, staring at tutorials that were only loosely relevant, value charts, and tables to learn what I have, and I still can't implement what I've learned fully.
A good thing to do is learn as much as you can about DBC Editing, which is editing the text tables in WoW's MPQs. Very fews guides on this exist, and the ones that you do find are for only the most simple of DBC edits.
There are still a lot of unknowns in the DBCs, and a lot of cross references, etc., but once you learn the nessecary aspects to it, you can do almost anything.
Code:
Create Flight Paths
Reuse old maps with new IDs, allowing you to have 2 completely separate dungeons from the same map
Change the Swear Filter Words
Create Spells
Rename Areas
Create NEW NPCS, and not just reusing old display IDs, but creating new skins, put certain gear on them, etc.
Create New Factions
That's a sampling of what you can do, there's tons of other stuff as well.
Good luck if you actually choose to learn the art of DBC editing, you'll need it.