5 Destinations The Doctor won't visit, (and 1 he adds to that number.)
The Doctor (10), Martha Jones, and Jack Harkness
Timeframe: Sometime after the end of Season 3 (Alternate Timeline naturally)
His previous incarnation would've let the world, nay the universe, fall to pieces, if only to make a little girl smile for a few more precious hours with a dad she never knew.
He isn't sure how he could've been so careless, now, looking back. He's sure that if They were still around, his ass would've been on trial faster then the Rani's mutant rovies on the president's pet cat. At the very least his previous selves would've had a syncronistic brain anyurism.
Then again, love did funny things to the brain, even a Time Lord brain. He blamed circumstance, maybe temporary insanity. Or that unsettlingly cute way her tounge poked out of her teeth when she smiled.
Nowadays he's a lot more cautious, at least about where he goes.
He won't visit San Fransisco on New Years Eve at the turn of the 20th century, no matter how much Jack needles him about "the best bar this side of the mustang head, Doc, their partys are legendary!" He isn't sure what meeting his seventh or eighth selves on that night in the middle of that muddle would do the timeline, but he's sure he doesn't feel like being the cause of the universe nearly blinking out of existance.
He won't visit Atlantis again, because he's been there twice in two different lives, and both ended in disaster. He's sure showing up before those moments would only cause trouble. Showing up after would be asking to be lynched.
He won't take them to Pompeii, and Jack agrees on this one. Never mind the fact Mel and his seventh self nearly got stranded when the TARDIS was nicked, or the ensuing predestined paradox with his fifth self unearthing said TARDIS in the 1980's. Jack had mentioned often that it was his favorite cleanup selection.
He remembers the Mawdren incident. Blinovitch was nothing but trouble.
When the TARDIS circuits get spot-welded and fused by another glitch in the systems, he insists they find Xeton-7 ore somewhere other then Varos. Martha protests naturally, saying Varos is the better choice, especially if they land sometime before it becomes high-demand. To this he has to add a very vehement not-in-this-regeneration-or-any-others, and shuts them out when his companions start asking questions after everything's settled.
Any time in the past that involves the age of the dinosaurs is also out of bounds, and when Martha finds him in the library fingering the broken shards of a golden star, she wisely doesn't ask questions. Just lays a hand on his shoulder, and gently suggests they "visit the Pleistocene era instead, I've always liked saber-tooth cats better then triceratops anyways." He still feels somewhat the heel later for refusing her birthday wish.
Two months later and they're running for their lives from a vampire he knows he delt with in his fifth incarnation, but obviously not yet, and Jack's trussed up somewhere by the local gangs because that whole mess with Capone obviously hasn't cleared entirely up, and he can't do a darn thing about it all, except rescue Jack covertly, avoid being seen or called "Doc" by his companions lest his seventh self get wind of it, and get all of their butts back to the TARDIS and get the frell out of dodge.
Later in the TARDIS, life's fallen back into normal frequency. Jack is helping Martha with her scrapes and wounds, Martha is fussing at Jack for not letting her look at his non-existant bullet scars, and he's in the library again, listening to their banter a few doors down.
He sighs to himself and crosses 1920's Chicago off his list of future travel destinations.
He might not have the same face, the same clothes, same body or attitude, but he's still the same man who would've let the universe fall apart for her smile.