Let's speculate, instead of whining:
1. The Fallen is the main anti-hero in RotF and, since he's one of the original 13 TFs (some sort of Judas) who betrayed his 12 brothers to side with Unicron, thus makes it a close possibility to see Uni in CGI (it's not live action, but animation).
2. So far we're only aware of the existence of the Allspark, but that doesn't mean that Primus and Unicron are not involved.
3. Robots as big as our planet could walk on the surface of a red giant sun.
Stop whine about everything that doesn't fit on our stupid insignificant marble as being too big.
Grow up!