Agreed on the awfulness of the trailer, mainly for the content. It is almost is a call to action for somebody to create a better trailer, which I might just do before 2011 ends...
I don't think SEGA would create a Sonic Adventure 3 for the 20th anniversary (or otherwise) because the series does not call for it. '06 was essentially Sonic Adventure 3 and Unleashed was essentially Sonic Adventure 4 (or a new Adventure series as evidenced by the Japanese name of the game). Both those games were misses ('06 was MUCH more of a miss than Unleashed was). While yes, SEGA has gone back to the 2D games with Sonic 4, I can't see a SA3 coming about.
If Sonic Adventure 3 did happen, it would be one of these two scenarios: Either the game would be a direct sequel to SA2 (making it a prequel to Heroes and Shadow) or it would loosly follow Unleashed and Colors yet would have the title of Sonic Adventure 3. Both of those scenarios are stupid.
Sonic Adventure 3's ship sailed when Heroes was decided to be the next game in the series. Rather than improving and expanding on what worked in SA2 and making an SA3, they went with the wonky team gameplay and simplified to story to the point that even Sonic the Hedgehog 3 has a deeper story than Heroes.
I think SEGA's best course of action would be to continue taking what works from a previous game, expand it in the next game, continue the naming convention of "Sonic (insert word here)" and once they find that perfect mix for the perfect Sonic game, start with the numbered sequels.