ooh good idea. i've seen pretty much most of the places i want to see - although for some reason the more i see, the more i decide i want to see...
i've seen quite a bit of america (at least compared to most brits!): new england, florida (although not the bit most people see, no horatio where i went
), las vegas (which was like being on another planet but a lot of fun, and i saw the canyon & dam there too), alabama, mississippi, louisiana, georgia, but i've never done the west coast and would love to, and there's a lot of the midwest i'd really like to see, including chicago.
the big things in the mid-east i wanted to see were egypt (most of it! i had a bit of an obsession) and petra in jordan, as well as jerusalem - i've seen all those now but would be quite happy to go again
as for europe, i've seen the usual bits, italy, france, germany, austria, (but oddly enough never spain or portugal, unless you count a week in the canaries when i was 4!), greece, turkey, bulgaria, poland, etc, but i'd like to see more of eastern europe (mainly because i LOVE the food), the balkans etc.
i've crossed russia (from belarus to vladivostok) by train but there is a lot more of russia i'd love to see, it's not somewhere you can do justice to in one trip really.
i did russia by train, and had a blast, it was an amazing way to see a place, so my next big trip idea is to take a similar train ride across the US. the way i see it, i've done one half of the cold war, i may as well do the other.
i've seen malaysia, and sri lanka but i'm not generally interested in india or the far east, primarily because the humidity really doesn't agree with me, it triggers my bipolar and makes me very depressed, very fast. there's a lot i'd love to see in india, thailand, vietnam, etc but i just think i'd be suicidal within weeks and not appreciate it. i'd rather see those things on tv and remain sane.
i'd like to see more of australia too - sydney is one of the coolest places i've ever been, i really loved it, and the tarmac at melbourne airport was really exciting too... but it's a huge place so i'd love to see more.
see, everywhere i go, i feel glad i've seen it and then think "but i haven't seen enough!"