The Mewtwo reveal? Really? You're counting a character doing a transition on a wacky Japanese Pokemon show as something?
...
Anyway, that's all small fries compared to what I can list off for Pichu ... but we can all agree that he's likely not coming back regardless and was appearantly gonna be replaced by the 3rd gen Pika rips.
As for replacing Lucario with a 5th or 6th gen mon I would say Victini is the best bet for a 5th genner (Masuda's favorite Pokemon + he succeeded in picking up the slack when Zoroark flopped. Also a packed in figurine not that I really think that matters). 6th genner I would see him just going with Newtwo and calling it aday. Or he could just go with Sylveon regardless of what happens with Mewtwo (ribbons could act like Ivy's whips before anyone questions how a quadruped could fight.).
And we're on the outskirts of a brand new gen ... GF is gonna give Sakurai that list of Pokemon they see as important to them currently and I doubt Lucario will be topping any 6th gen Pokemon they plan to push these games with.