"All the Presidents' Heads" could have made a pretty good two-parter, actually. It's a "big enough" story in scope to justify it, and they clearly had a bit too much material to cram into one 22-minute running time.
I agree. My biggest problem with the episode was the way they ended it; more time would've made it so much better.
Yeah, it would've been a lot better if they had enough time to properly work out the time travel mechanics of that one - there were a lot of cheats in the writing that they had to implement in order to execute it all within 22 minutes.