Wave 7 Predictions:
First of all, I'm predicting that in Rebels, the 7th Fleet is going to contain something new. Possibly a variant of the Imperial Star Destroyer, or a new Star Destroyer altogether. This will be included in Wave 7, because it's the 7th Fleet and it would be appropriate for 7 to be 7.
And naturally, Wave 7 will also be when we receive Grand Admiral Thrawn.
In terms of Rebels, it is far more difficult to predict.
I suppose we could get Bail Organa or possibly even Ahsoka Tano, as she had large scale leadership from behind the scenes as "Fulcrum". We are due for some new rebel ships as well, as Thrawn is seeking to destroy the majority of the rebel fleet, and Rebels could surprise us and show something like a new type of frigate that was destroyed before the battle of Yavin - which might be why the Yavin base only had fighters and no capital ships.... Just a thought.
And yes I know I skipped Wave 6, because we know that's going to probably end up being hammerheads and Quasars, with probably a ton more squadrons because ... well people love squadrons or something silly.