I just saw the movie last night, finally... here's my thoughts.
First off, I'm surprised at some of the confusion that people had about the Allspark.
It's ALWAYS been a part of the transformers history... it's always been the thing that brings life to new robots. All objects, like the Matrix of Leadership and the Creation Matrix (and Vector Sigma, and the like), are all connected to it somehow.
http://transformers.wikia.com/wiki/All_Spa...park_.28cube.29
Second... the movie fighting was done so perfectly, it was amazing.
Not just how the fights looked... but how they ended up.
First off, there was where Prime intercepted BoneCrusher on the highway. Didja notice how quickly he killed him? If Optimus had been present for the early parts of the next fight scene, they would have ended too fast, as Optimus would have taken them all out too quick.
If you want references to the varying power levels/fighting abilities of Optimus vs. Decepticons, go watch the original Movie, and see the scene where Prime arrives at Autobot city.
Not only was he taking constant direct hits from three different decepticons, which he just ignored, he was taking them out left and right.
So Prime couldn't be there when Devastator (the name they used for the Tank in the movie... I don't know WHY they didn't use Blitzwing, the triple changer who has a tank as one of his forms... but they used Devestator, with an alternate name of Brawl, which actually makes some sense, but is still an odd choice seeing as he's one of the Combaticons) arrived... he had to be occupied so he couldn't kill them.
Also, while Ironhide and Jazz could hold off Devestator, the second Starscream showed up, it was game over, and they got overwhelmed rather fast. Because Starscream is also way high up in the power order, which is probably one of the reasons people are so quick to forgive him when he backstabs them. Starscream also had to be given reason why he wasn't constantly in the fight, which is why he had to deal with all the jets showing up. Which was one hell of an awesome scene, the short dog fight between him and the jets.
Then Megatron showed up, and it was essentially game over. They gave him a pretty accurate power scaling of his fusion cannon, which temporarily took Prime out with one hit. This was also the time that Prime could show back up, without fear of overpowering all the other enemies.
It was perfectly done, even if they had to make up some enemies, like Barricade and Frenzy (Barricade for some reason was given Jazz's body, and Frenzy was just flat out made up, when they could have used something cool like Rumble, Lazerbeak, or Ravage), some of the Robots didn't make sense (Why didn't they make Jazz into his normal police car? Why was Scorponok a midget scorpion? Why give Brawl the nickname 'Devestator', when that will only confuse people?), and a few other problems.
It was so kickass, though... so kickass.
In regards to the robots who might show up... well, if he wants to go big scale, Bay will bring in the rest of the Combaticons (Brawl is already there), to have Bruticus form. Devestator could work, as Bonecrusher is already there too. Soundwave showing up would be awesome, as would Thundercracker or Galvatron. Galvatron is too much of a psycho to not be in there... though he really would need Ratbat to get the REAL psycho feel about him.
The Dinobots would kick ass, though I get the feeling that they would be fucked up somehow, with new ones being created intstead of just using the classic awesome ones. The Arialbots would be unlikely, as like the combaticons, they aren't as familiar (but unlike the combaticons, they don't have a member already there). Kup and Hotrod could show up, though I'd prefer if Hotrod didn't. That would mean theres too much of a chance of Optimus dying if he did, though Kup kicks major ass.
Prowl was one of Optimus' lieutanents, so he could show up. Jazz already died (he was another one of his main officers), so he could be a replacement there. Wheeljack is also wellknown, as is Brawn and Cliffjumper.
I'd personally like to see Ultra Magnus and Blaster show up... Springer and RC and Blur aren't quite as necessary, but seeing as they were all stationed to Autobot City (along with Perceptor), they'd make sense to be in the movie, too.
There are wayy too many options to choose from, frankly. Any and all would be good.
All the Witwickys made sense, though I really wished that they could have worked in the nickname "Spike" somewhere for Sam, seeing as that's all that they Autobots called him, from what I remember. It was odd, though, that Buster (the dad.. Ronald, or whatever they called him) wasn't a mechanic, though.
And the Hoover Dam... man, that brings back cartoon memories