Oblivion + Fable: Lost Chapters
Basically Oblivion was fantastic with its open world, guilds, quests, scenery, secrets etc etc etc (Of course it had major flaws too). What I would like to see added to it, is more RPG elements Fable can offer. Such as buying/selling/renting and upgrading houses, editing your appearance, good variety of voice acting and more interactions with towns people such as friendships/enemies and marriage. Oh and a living economy (of course there are mods for that though). Other features would include fishing in Oblivion and all the little fun things Fable had to offer.
Second is similar.
Oblivion + Mount and Blade: Warband
Number 1 reason is the combat. Oblivion's was horrid. Sometimes I get that urge to install and play Oblivion by just thinking about it... but once I do, combating monsters and foes just sucked! I want M&B Warband style, 4 way swings and 4 way blocks. Block at the direction he is swinging with your mouse to successfully block. That would truly benefit Oblivion.
Meanwhile Mount & Blade could benefit from the scenery, graphics(Great for its time), quests, voice acting, interaction with guilds etc. Both could really borrow ideas from each other.
Anyway my most favorite would then be three games :/
Oblivion + Warband + Fable.
Now it will take forever explaining what features would benefit who. But All three using the most popular features of each would be epic!