Mine would be along the lines of a pen and paper (pnp) RPG, shipping with a default campaign, and allowing for user-made addons and campaigns as well. In fact, the use of mods would be encouraged, and would probably be built into an updating system that would regularly scan a database for high-quality mods, addons, and campaigns (based on a user rating system or something), and then present these to you for approval.
I love the freedom pnp RPGs give. Having a comprehensive skill system (like the Star Wars D20 RPG), or basic skills with specialties (like the Serenity RPG), or even another system entirely would definitely bring far more freedom to a game. Every object would be usable, with tags pertaining to skills attatched to them for greater depth of interaction. While for a wooden bucket, you might only see the options "Pick up," "Throw," and "Fill with object" (meaning anything that can fit inside it, like water), if you had a Carpentry skill, you would also see the option "Dismantle." Dismantling it would then give you the objects it was made out of: wood, nails, and a piece of rope. This general idea can be applied to all world objects, and when you get into stuff like smithing, multiple object could interact with each other as well, adding even more complexity.
The inclusion of NPC AI akin to conversation bots like ALICE that would also randomly choose from a personality list, complicated by randomly assigned professions that would affect their interests and relations with the player would in and of itself give far more depth to the game than any other RPG currently out there. Combine that with (fairly) dynamic side quests (that can also be written and modded in by users), and you have an incredible game on your hands.
Part of the experience would also be developing your character; after finishing one campaign and defeating an evil wizard (or baron), you could move onto the next story arc with the same character.
Pnp RPGs give total and utter freedom; the goal of the game engine would be to supply the basics for that, and provide a few examples of what you could do, but the greatest part would be the user-added content, as I mentioned before.
Anyway, that's my dream game.
