Some suggestions:
Save or die should be replaced with save or unconscious. Always have a reason why any creature using such an effect would capture rather than slaughter immediately.
Use action points. Action points give the player a chance to mitigate weaknesses. They can use them to go for the heroic moment. And I allow my PC to use them to automatically stablize.
I'd argue that you shouldn't use gestalt. Part of the appeal of the 1 PC campaign is that the player, not the system, has to make up for weaknesses, either through recruiting friends, followers, hirelings. Leadership feat should be taken at 6th level.
You don't care any more about what's unbalanced. You don't have to worry about other players getting jealous. You don't have to worry about whether it will make monsters pushovers, because the PC still gets just one standard action a round.
Steal liberally from modules or published settings. In my experience, you'll be doing EXPONENTIALLY more work fleshing out characters. My one PC campaign is an intense, immersive experience. If your player likes games of diplomacy and social interaction and plays a bard or rogue or enchantress, be prepared for several sessions of role playing in a row. You'll need complex social worlds to play in. I feel like I've bought every module with a fully fleshed out town in it and I still want more of them.
Have your player set some ground rules. What are her boundaries? What kind of campaign does SHE want to be in: high fantasy, low magic, grim and gritty, survival, diplomacy, hack n slash, spies R us, romance?
If romance, post a separate thread on this board for how to DM that. Basic important ideas: give romantic choices with several different suitors, give romantic obstacles, make sure the love interest does not become a DMPC, have rivals for affections, have a BFF, include a ball or dance as an entire evening's skill adventure, get the parents involved.
If you use romance in your one PC game, do not LARP without warning.
Go shopping. Give out items that the player or her compadres can use or are cool. Better one cool item with some potions than three boring "realistic" +2 swords of genericness. Think about using ancestral weapons as a free feat to start the game.
Have memorable villains. In a regular game, their goal is to rule the world or some such nonsense. If YOUR one PC game, their goal is to rule the world and, in doing so, mess with your PC or their loved ones... Give a lot of thought to your adventure seeds and hooks.