What did the Civ 4 DLL source contain?
As concrete examples, with the Civ4 dll it was possible to implement/change
- the AI
- Introduce totally new concepts, like e.g. in FfH2 alignments and magic
- Create new XML tags (or even create your own XMLs)
- Somebody was able to add another yield and let it display properly
- Multiple maps
etc.
But like androrc says: You need somebody who can implement it. Not everybody is good in C++ or has the time (well, the usual problems).
But if this will be possible with Civ5 is a totally other question, and might also depend on other things, like if e.g. the engine can cope with such stuff, or what else the devs have hardcoded into the .exe.