If your happy with the Kef sound, then the logical upgrade path is to stay with them. I actually went on a similar path starting with the 2005's, I got the 3005's, then a set of q500's for the L&R's, then a set of r500's for the L&R's.
Their uniQ system makes them quite unique for imagine so if you wanted another brand you really should change all front 3 speakers to match; however they kefs are easy to mix and match making future upgrades fairly easy.
Depending on budget, I'd look at the q100's second hand which can be had for less than £150. They are a great speaker.
If your looking new, the new q150's are a step up.
The R100's are again another step up and imo, a great all round speaker for a surround system. I actually prefer the sound of the r range over the LS's, hence i sold mine and replaced them with the R300's standmounts. they are a tad warmer and more laid back than the ls's. This is obviously subjective and would advise a listen if you can to compare; however they are a lot easier to control which again is a benefit running off an avr.
The issue you will run into is that once you replace the L&R, you will see the centers limitations, especially at higher volumes.
I recently set up my mothers 5.1 and got a pair of Q100's off ebay (£130) and a center 3005se egg (£110) and once set up, sounded balanced and can vouch is a good LCR. Ideally you would want the q200 center but this is a big box and my mothers lounge would not accommodate such a big speaker.