What storylines? Autobots vs Decepticon on Earth has been done 3 times already, that war technicaly ended in Dark of the Moon, sure, there were various elements that were used but never properly explored, like the matrix of leadership, the various characters in almost name only, etc. But to be honest, that's been done and done, i wish i got to see stuff like Starscream actualy trying to constantly become the leader of the Decepticon, or Shockwave as a Josef Mengele type of scientist, but overall, many other doors are still open to be explored.
It's just like with X-Men, you have the main conflict between the team and Magneto, but there are still other stories you can pursue. The Fast and the Furious Franchise for example had 4 films considered bad consecutively, then it was able to give a more worthy 5th and 6th instalments.
Up until now, i think these films have had interesting ideas that weren't very well executed, just continue from what you started, but try exploring your themes better. Is Michael Bay realy the only guy who can Direct these films right now? They've been hyping space exploration, yet the only climaxes he seems to be able to do are those set in Urban areas, with the heavily promoted Dinobots barely getting any screentime. If anything, what the franchise needs is a new Director, somebody with new ideas, but with some decent knowledge on how to Direct action and more interest on characters.
I don't mind rebooting one day, but i don't think the plot is completely unsalvageable, you have all the right set-up, what you need is just better writing.