What would you buy next?

Get a wide angle prime or wide-standard zoom (which you have), and a telephoto prime. The wide angle will give you a larger DoF for any given aperture, so you can set the lens to hyperfocal distance (or whatever distance you like) and not worry about focusing every time you take a picture. A zoom gives you flexibility, and a telephoto for the times you're taking pictures from across the street.

a 10-22 like bohdank suggested could work, but I personally find that too wide.

St1ll wrote in post #6993628
Thx everybody; what I'm more interested in is portraiture, but not posed portraiture. I want to take shots of common people wandering around the cities of the world. That's why I was talking about versatility. 55-250 would be great for that...but at the same time I'd like to enhance the bokeh and reduce the dof to make the subject come out better.
Did you get the point?
That being said, is there a better lens accoring to my purposes?

For bokeh and shallow DOF the 55-250 won't cut it. Its too slow a lens. You should consider selling it and saving up for the 70-200 f/4L IS USM. It will give you the DOF and bokeh you want for the candid style you're looking for. It costs more tho. It costs about $1100 regularly. But, right now with the Canon Instant Rebate of $125 (until January 19th) + $200 off from the MS Live eBay Cash Back you can get the lens for $825 - which is an all-time low. Its a lens you will never want to part with - its one of the best ever, and hands down the finest 70-200mm Zoom ever made by ANYONE.

The other option would be the 85 f/1.8, which wouldn't require you to sell anything. But its not as versatile as the 70-200 f/4L IS USM of course. If you go the primes route, I suggest the 85 f/1.8 + 50 f/1.4. Because of the 1.6x crop factor the 50mm f/1.4 will give you the same FOV of an 80mm lens on FF - which is the classic "portrait lens" FOV (almost, the classic is 85mm). Meanwhile the 85mm f/1.8 will give you the same FOV of a 136mm lens on FF - which is considered a "short telephoto" FOV (almost again, the classic is 135mm). These framings are great for the candid style you're trying to achieve, plus the DOF control of these fast lenses will give you great isolation and lots of "pop".

Both options will cost you about $600 out of pocket. The 70-200mm f/4L IS USM will cost you the $600 out-of-pocket + the sale of the 55-250IS (which you will never want to touch again after you use the 70-200 f/4L IS). The 2 primes will cost you about the same $600 out-of-pocket with no need to sell the 55-250IS.

-Alan