Looking at the OP's current gear list I think that whilst the 18-200mm lenses would give him the versatilty that he seems to be after I can't help but feel that it will never get used. He already has a 55-250mm lens and an 18-55mm so essentially unless he is flitting over the 55mm boundary a lot I would think those two lenses should give him more than enough.
I get the feeling that the OP might have some money burning a hole in his pocket with a desire to spend, but not sure what he wants save that he want something for the camera.
I, personally, would suggest looking at a good flash - you can get a 580EX2 for $395.00 (amazon price) which is currently top of the range for canon. You could also look at ringflashes if you want something more dedicated to macro or maybe portrait work.
In the end though my advice is to sit down and work out two things
1) What is it that your current gear is unable to do for you - if it does everything you need then consider which aspects you would like to upgrade to "the next level".
2) Research about those limited areas and get an idea of what's on the market that will let you achive those goals - look at the low, mid and highpriced items and then decide on the amount you are prepared to save toward to see which you can really go for. Sometimes its worth it to wait and save a little longer and get something that is worlds better rather than to buy fast and get something that will always be a "lesser" tool