I don't know if there really is any room for a big surprise these days. Things almost guaranteed to have:
- Semi-metallic build like the Note 4, if not full metallic
- 1440p AMOLED (I'm guessing 5.1-5.2 to keep the size from getting too large)
- Newest SOC
- Lollipop out of the box
- Camera at least as good as the Note 4
- Toned down TW (progression of the Note 4).
Things that may or may not happen:
- Minimal bezels like the G3 (or not if Samsung sticks with IP protection. If so, possibly an improvement over the size increase from the S4 -> S5)
- Slim while increasing battery life (can Samsung improve their packaging any further?). Possibly Z3 level battery life, which should be fine for near everyone. I don't expect a huge step forward like the Z3 compact.
- Stereo speakers, though I'm 100% sure not front-facing
- Improved fingerprint scanner - I'm ok with the Note 4 scanner, but it's not as good as Touch ID. Huawei showed that someone besides Apple can do a great scanner
- 32GB standard storage
Honestly, outside of a dramatically refreshed TW (which the Lollipop builds of the S5 don't reflect) or some miracle of packaging that results in a very svelte, huge battery life device, I don't expect anything "wow", but very good.