... find Oblivion simply requires more imagination to roleplay. ...
Right. Fortunately or Unfortunately, I find roleplaying in Oblivion to take imagination and restrictions. Personally I think that's unfortunate, but some might think otherwise.
So there are some common restrictions. For example, if you have a good character, only do good quests. Not the Dark Brotherhood, or some of the Daedric quests for example. Maybe visit the nine wayshrines on a regular basis. For an evil character, maybe live in the Vile Lair and do evil things.
Maybe restrict weapons or armor. Like only use Destruction, or never use Destruction.
Maybe have a personal quest, like being a Goblin fighter. Perhaps with a back story (eg Goblins killed your family).
On the PC there are mods that require eating, sleeping, and have realistic fatigue (you get winded if running up a steep hill or swinging a heavy warhammer). Also other nice roleplaying mods. Like the Linchal Home for necromancer characters.
Also maybe restrictions on stats. Like if you have a mage, never get strength up to a high level. And never go beyond a certain Chameleon or Reflect Damage percentage.
The game is an open sand box. Perhaps too open. But you can provide the boundaries.