If a new trailer DOES come out at the end of April / start of May, it WILL be with Dark Shadows. There is NO way that WB would release the final trailer to their most anticipated Summer film of 2012 with The Avengers. That is not going to happen, full stop.
It all comes down to interest, that leads to money. Releasing the final TDKR trailer in theatres before releasing it online is obvious (given BB / TDK / TDKR marketing history), and will lead to more people watching a film at the cinema. So who do you think WB would rather get more money for? The Avengers (the competition), or a WB film?
I know that the prologue was released on Imax with MI4, but that was due to necessity (it was the ONLY Blockbuster Imax release in December). And the 1st full trailer was with Sherlock Holmes 2 (A WB film).