I've been working on a game engine, which offers both players and scenario-editors nearly unlimited possibilities in terms of player-world-player interaction.

The project interface consists only of command line and text-output and it works on the .NET 3.5 libraries, so even the XP users will be able to play/edit the game.

My main idea is to make the engine support many different languages through allowing the players-editors to modify the engine system variables, such as commands, system messages et caetera...

The second idea is to make the basic version of the game freeware, hoping GOG to add it to their library of free games, which may be nearly impossible, as the game contains no graphics or sounds (the online/commercial version will add these features)

If I am to be understood:
The main, "original" scenario(s) will offer both visual novel style, RPG, classical text-based adventure, logical riddles and even some crafting.

Moreover, one will be able to share their crafted items, creatures, locations... full scenarios and other stuff just by giving someone a properly formatted text file with those things imported from the base application.

I expect you've studied the Infocom Parser and Z Machine in some detail, as their environment was the watermark in interactive-fiction for a lot folks (myself included).

As for development platforms, I would actually suggest building this in a scripting language, such as Perl or Python. That will be eminently portable, will run on just about any platform, and be very friendly to others who want to contribute to the project. You don't need machine-language level speeds for a project like this, so developing it in a compiled language actually limits you in several ways that you don't need to be limited.

Up to you though, it's your project, and you have to work with what you're comfortable with. Good luck with this! :)