Welcome to FD
Craig,
Compared to some, I'm relatively new to the FD line. I've been shooting with it for a few years now. Over that time, I've experimented with just about everything except the 14mm, 200mm macro and 600mm. As Gnashings said, what do you want to do that you can't do with your current kit? I shoot mostly landscape or wildlife. I love my 24/2, though the 24/2.8 is much cheaper, easier to find, and is also a great performer. I often carry the 20/2.8 on outings as well, but it takes some skill to use the distortion in such a way that it isn't a nuisance. I like birdwatching. I'm not the rabid type that is armed with an Audabon book and 20 other AARP members pointing and yelling. I just like colorful birds. I have a 300/2.8 Fluorite model (from the pre-L lens days) and 1.4x teleconverter. I can walk around with this and get some decent shots of birds that would otherwise be unable to get due to them being frightened away. The 1.4x has a lot less quality loss than the 2x, IMO.
35mm tilt/shift? Get a view camera, unless you're hindered by space requirements. Low end monorail view cameras offer larger negatives and more movements for about the same cost, if not less.
Some rave about the 85mm (both 1.8 and 1.2) for portraits, but you'll rarely find people in any of my photos.
As much as I liked the 100mm macro, I'd rather be shooting wildlife. When I had this lens, I wasn't into squinting into my eyepiece while crawling around on the ground, or trying to hold something to either reflect light, or shield wind. I tend to be distracted easily, so it was hard for me to stay in one spot. I've since found large format and ogling the ground glass is ok if I can stand up doing it.
I'm a big fan of the 135/2.5. It's a sleeper. I find that not many folks use this focal length. I got mine for $60 in mint condition and have got my money's worth several times over. This and the 24/2 are probably my most used lenses.
The 400/4.5 is nice, but I've never owned one. The one I borrowed was the newer FD model. Get a nFD instead of SSC if possible. The newer FD model has a different light baffle in the back, allowing use of the 1.4x TC. I've heard the SSC has a light baffle that prohibits the use of the 1.4x, but the 2x will work fine since it doesn't have the protruding lens element.
The 500L lens is great. Heavy, so make sure to use a tripod.
600? Never used it.
The 800/5.6L is an amzing lens for birdwatching, but you'll find out real quick that a 10 pound lens requires a tripod, and a healthy one at that. My little Bogen 3021 wasn't enough so I stepped it up to a 3046. The tripod isn't light, nor is the lens. Add in a camera bag, some food and water. You'll soon find yourself working from the car instead of hiking out into the wilderness.
Search "CanonFD" in Yahoo Groups. There's a lot of info in the archives. Look before asking questions. Since the FD line is no longer being developed, your question has probably already been answered.
*whew* Maybe my comment seems like a pro-FD rant and should be redirected to the soapbox.
Welcome to the wonderful world of FD!
-Chris