Interesting concept. Too bad it would be so hard to implement, because I think adapting a non-IF video game to IF would normally be a nice “exercise” for making IF.
I’ve noodled around with adapting a graphical RPG to parser IF, but the game I picked was Dark Sun: Shattered Lands. Definitely a huge difference in scope between that and Oblivion. Even so, I imagined a quality adaptation of Dark Sun would require oodles of rooms and oodles-upon-oodles of objects. Specifically, I’m making a rough estimate of over six hundred rooms. I’m not even sure how well tools like Inform can handle that much stuff.
Anyhow, this is fun to think about, so here are some totally hypothetical design thoughts I have: (focusing on fun factor with almost no concern for the practicality of implementation)
A Kerkerkuip-style combat system could be nice, but you probably want to downplay the combat in general (spend a smaller portion of game time fighting than in graphical Oblivion, maybe make the combat easier than Kerkerkuip) and focus more on other qualities.
Definitely don’t try to faithfully reproduce the floor plans using the IF room system. It will be way too many rooms. Also, don’t try to make everything to scale. Maybe a house needs five rooms but an open field that’s the same size as the house only needs one room.
As for the scale of the open world areas… hmm… maybe the shortest path between Anvil and Kvatch (for reference) should have about five to eight rooms. Make the areas feel big and exploration-y partially with the room description prose but also having a lot of interactive stuff–even just a bunch of examinable scenery objects which mention more examinable nouns in their descriptions. (And of course, you need to collect crafting ingredients.)
My first impression was that the rooms for the open world areas should usually have exits in all directions, but the more I think about it, the more unpleasant that sounds. Maybe better to redesign the map as a series of interconnected paths.
Implement “fast travel.”
In addition to fast travel, maybe some kind of Nightfall-style “go to” command would help match the navigation convenience of the graphical map and ever-present mini-map.
Is there anything that IF is good at where Oblivion could use improvement? If yes, embellish that. For Dark Sun, I was thinking NPC interaction. Nothing comes to mind of Oblivion though.