I had always have my 50mm with me but have been thinking to get a 24-70mm which will kinda of make the 50mm (which is a f/1.4 AIs) redundant for travel. I think my future set up will comprise of 20mm, 24-70mm (akan datan) and 80-200mm.
If you are into architecture and landscape, a 12-24 or tilt-shift lens.
If you are into Safari and wildlife, a 600mm lens
If you are into portraiture, a 50/85/135mm lens
If you are into portraiture and action, a 70-200 f/2.8
etc etc etc
Your own focus and style will naturally already lead to an answer to your question. There is no such thing as "die die must get this lens no matter what you shoot".
fisheye lenses for full frame and DX
10-16 lens for DX
14-24 lens for full frame
17-55 lens for DX
24-70 lens for full frame
70-200 lens for both
60 micro lens
105 micro lens
2X tele convertor
most already have, some in wish list, some will rent only when needed.