personally,this is what "I" would like to see...
3 years later...Sam is ready to graduate college (i figure with some of the stuff from the Allspark in his head,he could ace some of the classes),with Mikaela by his side,as they've moved into an off-campus apartment together....
after the "unveiling" of the TF's,the reaction is mixed: the Autobots have helped humankind with advancements in technology,while still suffering Decepticon attacks throughout the years...
indeed,one of the bloodiest battles is taking place in Siberia (TF's in the snow!) at the beginning of the movie when the Autobot's are winning,and suddenly the battle is interrupted...Astrotrain has arrived from Cybertron,barely alive,to tell a horrific tale - Cybertron is no more! consumed by the Chaos God, UNICRON! (this would be shown with the laser-light show from the eyes,ala Prime in TF1 )...Megatron,in a rage,commandeer the Decepticon's ship and flies there at full speed (it seems he DOES care for something...his homeworld,and nothing else)...confronting Unicron in deep space,he attacks,but his ship is blown apart,and Megs killed...that is,until Unicron finds a use for him and reformats him into his seeker,GALVATRON! he sends him to Earth with one explicit purpose - get the Matrix of Leadership, (carried by Optimus Prime,taken from the wreckage of the pyramid at the end of TF2),whic he fears...
suddenly,the now leaderless Decepticons are forced into an alliance with the Autobots to defend themselves,and,like it or not,the only planet they now have...upon this story we will have the usual commitment issues of Sam and Mikaela , humor,Starscream's golden opportunity,with both the #1 and #2 Decepticon's out of the way,to become leader (and all the backstabbing that will come with that),and Sam Witwicky's true,ultimate destiny...after all,"HE" earned the Matrix,not Optimus,and will find himself thrust into the greatest galactic battle of all time,against his will,as only he can save his planet,as well as the remainder of the Cybertronian race,from the Devourer of Worlds....or die trying...
Heroes will die...Villians will fall....Evil will rise...and the Chosen One will emerge
....of course,i'd also have this be the LAST movie!

t: