So with Alpha Trion being ruled out (not 100%), the most popular two possibilities for the the Bot from the DOTM teaser to be is Vector Prime or Sentinel Prime. Here I'll discuss only the SP possibility. There are some strong facts about it.
1. (In Transformers 2007) Did you know that in an early draft of Transformers, time stops as Sam falls down the building Megatron smashed, and Sentinel Prime appears to him via the All Spark. Sentinel would've clarified the history of the war and the relationship between Optimus and Megatron (when the writers intended them to be siblings), and gave him a clue to defeating Megatron with the Cube.
2. (In ROTF) Sentinel Prime led an expedition where it took him fourteen galactic convergences to get an echo of something. Whether or not he is one of the original Primes or in the later Dynasty of Primes is uncertain. He does however seem to be one of the direct ancestors of Optimus Prime. Sentinel Prime was mentioned in a rambling lecture of Sam's while under an All spark fragment-induced trance.
3. (In DOTM filming) According to a guy on set the Fire Truck is Sentinel Prime and he is Optimus Prime's father (matches the theory from ROTF that he is a direct ancestor of Optimus). I don't believe that Sentinel is the truck but I do believe that some guys really heard the name Sentinel and that he'll be in the movie.
Also the striking similarity between the Bot from the teaser and Peter Cullen might also point that he is a relative of Optimus.
So a final conclusion: Sentinel was in the draft plot for TF1. Sentinel was mentioned in TF2. Now I really can see him appear in TF3.