Just for the sake of cataloguing, I made a mind map of a comprehensive small-format camera system. Components in black are things I already have. Those in blue are things that I either eventually want more of, or eventually want better versions of. Things in red, I don’t have at all. Some of the items listed are general (‘lens hoods’) while others are specific, like a particular studio flash unit. Almost certainly, I have overlooked some things.
Clearly, there is a lot of expensive stuff you can use in combination with an SLR or dSLR body, and this listing doesn’t even include chemical or digital darkroom supplies. For those who aren’t wealthy, assembling a good fraction of this stuff is a multi-decadal project.
See also: Lens selection survey
A 28mm prime lens definitely would be a nice addition to your Rebel. You can buy the nikon version for about 100$, but it’s only F2.8. I think nikon used to make an F2.0 but it’s MF. They probably stopped to encourage people to buy the absurd 28mm F1.4
My ideal set up would be something like a D90 (or whichever the cheapest nikon DLSR with an AF motor in it is), with a kit zoom, and 28 and 50mm AF primes, and the 70-210 F4-5.6 AF zoom.
http://www.kenrockwell.com/nikon/d90.htm
This camera actually looks pretty awesome. Also, it would go well along side my F90x, having the same number and all.
Among the expensive things, I think the 70-200mm zoom would do the most for letting me try new kinds of photography: wildlife, concerts, etc.
Given the state of the global financial markets, and uncertainties about the future, lenses that should last at least 20-30 years seem like a relatively good investment.
I bought an LP120 manual flash, along with a light stand and shoot-through umbrella.
I now have two flashes: one which I can activate using the CyberSync radio triggers, and the other of which can be used as an optical slave.
I also have three rolls of gaffer tape: two 60 yard rolls of 2″ wide tape, and one small roll of 1″ wide tape.