I currently have an eleven year old Canon Digital Rebel XT and I would like to upgrade to a Canon something. I intend to use the new camera for mostly taking pictures of my family and sometimes of wildlife and sometimes pictures of cities and places that I go on vacation. I would like this new one to last me about as long as the old Digital Rebel XT did and needs to be a Canon since all of my hardware is also Canon. My budget would be up to $1,200 and I would like to see what forum members think my next camera should be.
When asking for a product suggestion on most forums I've found the answers you get are quite often the models the responder would prefer to own. They are not always the model you should choose for yourself. Of course, since you have Canon lenses, the suggestions will be to simply upgrade to a new, shiny Canon. That's sort of a no brainer.
I still have my XT, though, since I haven't used it in several years, I'm currently looking for something more basic and more portable at a fraction of your budget. So other than saying stay with Canon unless you want to buy all new lenses, there's no model for me to recommend. Of course, at your budget you should buy the best Canon, no? Then you won't suffer from camera envy in the near future. Then you can't blame the camera for the results.
IMO you've listed several items which you do not own that would likely make a noticeable improvement in at least some of your shots. A tripod/monopod first among them. I've owned several Rebels - the XT was the first DSLR - and have found them to be all the camera I need in 80-90% of the situations I would shoot. There are workarounds that make the Rebel perform as a more expensive model such as external flash and remote shutter control. If you're not looking to sell your work, IMO, a Rebel will seldom be the camera which limits you.
The question was asked, (paraphrasing) what is it the Rebel does and doesn't do that you think is important? Until you know why you are changing equipment, you have no target for what to look at. Are you going to be trading the Rebel body in on the new camera? Or, using the XT as a back up? What are your priorities? This is typically the most difficult question to answer because many people don't think in terms of priorities. Your needs and wants often revolve around how you will be using the equipment and how you perceive the results you achieve. Is it important to have lightweight? Is it necessary you have all the newest geegaws? Do you want to control the camera or have the camera control you? Once you set these priorities, you can move on to results. What sort of photography are you interested in? Stop action sports would be a slightly different idea than macros of bugs. Though, if your goals were to capture those bugs in flight, they might be the same. Simply buy the top o' the line and the camera itself probably isn't the limiting factor.
At your price range, it's possible you should move beyond a DSLR and look at the next level of cameras. Do you think the DSLR format can still satisfy your needs?
Your initial question is sort of like asking which wine to buy or which car to own. Very broad questions can only be answered with very broad, non-specific responses. Or a suggestion to buy what the poster would prefer to own at that price range.
Can you narrow your priorities down any more and tell the forum what you think the Rebel isn't doing and what you would like your next camera to do for you? And in what situations the camera would be used?