Assuming you are sticking with your 30D or another crop sensor camera, I'd recommend a 50/1.4 and an 85/1.8 for portraiture. They are partcularly nice for candid work, where a big black or white zoom can be more intrusive. If you want to do wider, environmental portraits you also might conisider a 28/1.8, which can double for a walk-around prime... it's reasonably compact even with its matched lens hood.
If you prefer a zoom for a walk-around, there are two or three schools of thought:
1. A large aperture zoom, probably preferrable if you don't have some faster primes. A 17-55/2.8 might be the top choice here, but if you already have a 24-70/2.8, you've already covered this base.
2. A compact, versatile zoom that allows you to travel really light.... the EF-S 15-85mm is a good example. Here you would likely want/need some large aperture primes (such as the 28, 50 and 85 mentioned above) to complement the zoom for other shooting situations.
3. A budget zoom... the 28-135 IS is a bargain on the used market, and the EF-S 18-135 is only about $100 more used. These also usually require one or two or three faster primes, for particular situations.
What you are lacking is anything particularly wide. You may or may not need that. The 15-85mm gets pretty wide... But Canon's 10-22mm is a lot wider and there are several other good choices from the third party manufacturers. Of course, maybe you just don't use a wide lens very much.
Telephoto like the 70-200mm would replace your 55-250 with a better built, better sealed, more durable, faster focusing lens... possibly a faster aperture if you go with an f2.8 (bigger and heavier tho)... not a huge difference in image quality, though.
If wanting to shoot wildlife/birds, 200mm and 250mm comes up a bit short. There are some 70-300 zooms, ranging from cheapies to expensive L-series. But a 300/4 IS with a 1.4X teleconverter or a 100-400 or 120-400, 150-500 might be needed for smaller and shyer subjects. Of course, maybe you don't need a longer telephoto, depending upon what you like to shoot.
You mention macro and there are many good lenses to choose. For now, you might just get some macro extension tubes and use them with your existing lenses. Depending upon the combo, you can get pretty high magnification just with tubes.
There are also tilt-shift lenses for architecture/landscape (and some macro), fisheye for wilder effects, and other specialty lenses. You only mentioned wanting to do portraiture, might want to develop a list of other interests and where you feel your current gear is lacking... that will help guide you planning for your lens kit.
