* A core engine that understands the code you write.
* The ability to run this engine in the background.
* A unified API that would make possible for plugin authors to interact seamlessly with that engine but also between each other.
* Certainly a thousand of minor details.
You can add as many fancy plugins as you want, you'll never get an IDE.
You'll get something that looks more or less like an IDE and works more or less like an IDE, but not an IDE because an IDE is not a text editor with a file explorer on the left, a class explorer on the right and a list of compile errors underneath. It is much more than that and Vim doesn't have and can't have that "much more".
Unless you count external compilers/debuggers/profiler/linters as that "much more" but that's still not an IDE. A DE, maybe, but certainly not "I"ntegrated. Well, not even an "E"nvironment, actually, given Vim's design principles.
I always forget when i've started these discussions sorry for the late reply.
That's a pretty narrow definition of an IDE, especially since the history of IDE's goes back far before a time in which any IDE could be said to have a core engine that understands the code you write. In fact that "requirement" is almost entirely driven by the rise of Java and the ridiculous amount of code it forces you to write. Prior to Java IDE's I can't think of a single IDE that could be said to have such an engine, including Visual Studio who's features in that regard were driven by competing features in the Java world.
The 2nd requirement is obviated by the lack of the first; the 3rd and arguably the 4th are already present in vim. An IDE, at its core, is exactly what you claim it is not: a text editor that provides navigation features (possibly but not requiring a class navigator—you seem quite focused on object oriented features) and an ability to run the code under discussion. Emacs very definitely is an IDE under the limited definition you provide, including an engine for understanding the code you write.
I'm also not sure about why you think external compilers/debuggers/profilers/linters are disqualified from being the core engine you're fixated on—you realize that every IDE out there does exactly that, right?
