Figured I might as well add in my two cents.
With the mention of the Dinobots showing up later in season 2 of Prime, I put Grimlock at the bottom simply because it's expected for him to show up.
While not having appeared in WFC, he played a big enough role in Exodus to warrant an inclusion in this list. Plus, his getting a Prime-style MMO body increases his chances of showing up eventually.
8. Omega Supreme
If not in the present, then flashbacks will suffice. This guy was of a great importance in both the games and the novel, so an appearance by him in this show would only justify his character further.
7 Sentinel Zeta Prime
In flashback form ONLY! He is dead and must stay dead. But, it'd still be nice to see him appear on screen if only it were in a flashback. After all, he was the immediate predecessor of Optimus.
6. Air Raid
These guys were AWESOME! They had GREAT personality and played off each other just so wonderfully. Air Raid's headstrong and has a bit of an attitude, while Sideswipe is sarcastic and battle-happy. They make a GREAT pair.
Former best friend of Starscream and member of the Seekers, who wouldn't
want him to reappear and reignite his rivalry with Starscream? Especially with all the drama Starscream has and is still currently going through? It'd be interesting to see Jetfire reaction to all the change that has happened to Screamer over the years.
And old friend of Optimus' and one of his strongest soldiers, he'd be interesting to see interacting with both Ratchet and Bulkhead. not to mention that he's getting a Prime Cyberverse toy, so he pretty much already fits in (even though that doesn't guarantee him an appearance in the show, it's still gives hope for such an appearance).
2. Alpha Trion
Optimus' mentor and father figure, as well as one of the Original 13 Transformers and author of the Covenant of Primus. I'd think he'd be important enough to deserve an appearance. Even if only in flashback form would do.
Why Jazz in the number 1 spot? Because he's Optimus Prime oldest and best friend! They were Cybertronian BFFs who shared drinks together! Jazz IS Prime's closing thing to a brother (aside from Megatron, of course). And he just might be the only one who can bring out some real emotion out of Stoicmus Prime.
And, that's my list.