Tougher Knights: I think that The Questoris and Dominus chassis will both get the -1 Damage rule applied since Knights are more fragile than ever in the hyper-lethal 9th edition. Not sure if Armigers will get it or not.
Stomping Nerfed: I big surprise in the Eldar codex was to see how much the Titanic Feet had been toned down. I fully expect this to be rolled out to Imperial and Chaos Knights meaning that if you want to hit properly in melee, you will need a dedicated weapon instead of just kicking stuff. I expect the Reaper Chainsword will get a sweep attack mode for dealing with hordes, not sure about the Gauntlet.
Better melta: I think we can expect the assorted melta weapons to get buffed. Some of the big ones like the Thermal Cannon might even get +2/+4 Damage at long/short range to emphasise just how heavily they vapourise stuff.
Better plasma: I think that the Plasma Decimator on the Castellan will be buffed S8/9 and D2/3 in normal and overcharged modes respectively. This weapon is currently almost always fielded as the Relic version and after the buff to the Suncannon, I am confident Cawl would never be willing to be outdone by a bunch of pixies.

Any other speculations? Ideally stuff informed by what we have seen so far rather than just guessing or wishlisting.