I would like to see a metal casing on the S5 personally, though if the rumours of the F series are true then I guess it just offers everyone the best of both worlds.
Aside from that I think I want them to bring in a new shape, and some more of an individual identity to the phone. I complained for so long that iPhone's all look, feel and act exactly the same as each other and it was a pro on Samsung's side. But then the S4 felt like the S3.2 rather than an upgrade.
I'm not sure it all matters for me though. I have myself an S2 which I bought in January 2012. I had to spend £150 on a screen repair earlier this summer (it fell out of a hole in my bag, bad luck rather than phone quality) only for the SIM Card Reader to die on me just over two weeks later, and then last week for the screen to die on me entirely so it's blank. I'd call it a glorified mp3 player now, but even bargain mp3 players have a screen I can see, so it's not even glorified
I think after 10 years with Samsung phones I'm switching to Sony for my next one.