Hey there, I'm an 18 year old from New Zeland. I love the Transformers franchise; I first became aware of it midway through the RiD cartoon series. I thought the Unicron trilogy was alright, and some parts of G1 are okay too, but I dislike the Beast era and Animated. But the Michael Bay film trilogy and Prime are probably the best things to happen in the franchise.
I am also a contributer to the Primeval, Almighty Johnsons and Matthew Reilly wikis.
A fourth film
As much as I would love to have another live action film, it would clearly be difficult to come up with a good story. That and so many of the main Decepticons (Megatron, Starscream, Soundwave and Shockwave) are now dead. I mean sure, they could resurect Megatron again, possibly into Galvatron, it would just be that they would need a new method, that or have a completely new Decepticon leader. If it were me writing, I'd just have it that before Megatron died he backed up his memory into Igor, who would go on to create a new body for his master and thus resurrect Megatron (kinda).
Unicron would be a nice addition to the franchise, and it has been discussed before, and the animators probably would be willing to take on the challenge of such a gigantic robot, escpecially after Devastator and the Driller. I'd definately like to see some flying Autobots thrown into the mix, new combiners, and triple changers (now there's a challenge). Throw in the death of some human characters, add a visit to Cybertron (if it still exists anyway) and explain the Allspark's and Transformers' origins, and... well, who knows?
And much as I'd love to see a completely robot-based story, that's not likely 'cos otherwise the critics'll just have a field day complaining. So keeping all that in mind, they will either have a fourth film, or they'll just give it a few years and reboot the whole franchise.
Transformers Prime Optimus's memories
While the ending of Transformers Prime season one was very dramatic, I found that Optimus losing his memories after using the Matrix of Leadership to extinguish Unicron's spark completely startling. However, I believe that restoring Optimus's memories is going to a task, not for the Autobots, but for Jack. During One Shall Rise Part 2, Optimus failed to answer Jack's question as to what would happen when he used the Matrix, and yet later approached him to give him another Cybertronian relic, the Key to Vector Sigma.
My guess is that Optimus knew what would happen to his memories in advance, and planned to have Jack restore them using the Key, trusting him due to his growing maturity (as he did note) and the similarities between them. But Optimus likely did not expect to be tricked into Megatron's lies in this condition. Thus it might be up to Jack and his responsible nature to find Optimus and save him from the Decepticons.