Well, Siege is the first part of a three part series, just like Combiner Wars, Titans Return, and Power of the Primes was a three part series, with each part running across a single year.
Siege is part 1 in what's being call the War for Cybertron series, with the basis of Siege being their last day on Cybertron before heading to earth like in the cartoon, and is running for 4 waves most likely, with 51 figures total given on the basis of a Hasbro website with 51 slots for figures in the line for 2019. And they've only revealed around 40 figures.
We know things like a Generation 2 repaint of Megatron is happening again as an online exclusive, but has not been revealed yet. We can expect about roughly ten more figures that have not been leaked or revealed yet, and I'd guess they would be characters from the 1984 and '85 lines. I'm expecting a bunch more Autobots since there weren't many more big Decepticons in 1984. Probably a Bumblebee in there on the basis of recent art from IDW.