Well, LUA... I don't know anything about it actually But I know World of Warcraft has extensions / plugins written in LUA, which work fine.
But it is a script language I believe, which I hate on principal.
Interesting, but considering that much of the web now is built on technologies like Javascript and Flash (which uses ActionScript which is a derivative of JavaScript IIRC), I don't think scripting languages are something to be dismissed outright.
Fortunately in principle, you really only need to support one plugin system written with a general-purpose programming language, because the plugin can itself support the ability to host and run other plugins that are written in other languages. For example, imagine that your plugin system supports DLLs. I can then (hypothetically) write a plugin DLL that actually is a LUA interpreter, and can therefore be set up to actually carry out the game mechanics by running LUA script code. And then a year later maybe I can add support for other scripting languages in my plugin DLL.
Anyway, with that in mind, perhaps it's sufficient to start with supporting DLL and Delphi packages (I'd start with Delphi packages as presumably it would be easiest for you to test the system with, seeing that you are developing the system itself in Delphi). Or maybe both--no one says the system has to support only one option.
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Platform independence is, to be fair, a tricky matter in general. Scripting languages do tend to be more platform agnostic in general. Languages that must be compiled to native code are generally not as platform independent, and even when they are at source-code level, you still have the issue that the resulting binaries are always platform-specific. Java is the only thing I can think of at the moment, that is not an interpreted scripting language, but also does not compile to native code, and is explicitly designed to be platform independent. It does require the user to separately install the Java runtime for their given platform though.