It’s an interesting idea, it’s just doing things sort of in reverse.
Usually extensions (as monikered) “extend” the vocabulary of the game engine for someone writing a game to add more things it will do…shortcutting the author from having to write code for a process that’s difficult or complicated.
You certainly could write a killer engine as game code, then plug rooms and details in with an extension. I would imagine some people might do this on their own for some specific reason. The problem is this is totally backwards for an end user. They would need to obtain the Inform 7 (or whatever system) IDE, understand how to install an extension, then play the game either in the editor, which allows them quite a few cheats and cosmic powers, or compile it out for themselves for play using an interpreter. It sort of makes me think of that episode of THAT 70’S SHOW where Kitty and Red Foreman go to a restaurant with a new-fangled salad bar and he throws a fit saying “If I wanted my wife to make her own salad, we could have stayed at home!”
If you manage to make a really neat set of integrated systems for conversation, combat, magic, all that…it would probably be more of a benefit to release that as an extension, so you and other authors could make use of it. Your goal would be to make a suite of utilities that altogether are more useful in concert (or possibly simpler to use, or exist in less of a memory footprint, say so the game could have lots of features and still compile in Z-Code) than any other handful of the stalwart extensions that already exist.
For example - I tried using Output Filtering to save myself having to correct a bunch of line breaks and capitalization issues, but that extension is a bear, requiring Text Capture and slowing my glulx project down unacceptably with all the other things that were going on.
One type of extension that might be useful would be one that allowed an author to define RPG like statistics and easily manipulate them in a game with dice-rolling mechanics that incorporated magic and combat so you could do something like:
Cave is a room. A troll is a kind of aggressive monster. A troll is usually level 2. There are three trolls in Cave. A troll usually incorporates a damage bonus of +2. A troll usually incorporates an attack bonus of +1. A troll defeat usually grants 100xp/Player Level.
A club is a kind of weapon. A club usually causes 1d6 damage. A club usually hits with a 3 on 1d6.
A troll usually carries one club. A troll usually loots 1d10 gold. A troll sometimes loots a treasure map with a 2 on 1d20.
MegaTroll is a kind of troll. MegaTroll is a boss monster [giving bonus XP]. MegaTroll loots 3d6 gold. MegaTroll incorporates a damage bonus of 1d6. MegaTroll incorporates an attack bonus of -2 [due to his only having one eye and no depth perception]. MegaTroll loots the Amulet of Awesomeness. A MegaTroll defeat grants 300xp.
Your idea, though, is ambitious, and I’d love to see what you come up with!
Too early to say more though.