I believe there will be a Poison type Legendary in Generation 6. If not, it'd be a huge disappointment. When it comes to the final Generation, I'd love to see like, the ruler of all specific types considered as Legendary Pokemon. But I feel they may screw up the appearances if they did that so probably not a good idea.
The only trainer we've faced with a Legendary Pokemon is N. Should there be more trainers in the game's main storyline with Legendary Pokemon to battle against, such as Elite Fours or Champions?