A bit random, but I would like new Pokédex entries for the old Pokémon. Rewording the old information was never fun.
I'd love this too - give us a better chance to not only learn new things about them, but to reconnect with them on a new level, especially after them being more discluded in the most recent games. Also, would be especially good if Gen 6 followed more in suit with Gen 4 in terms of the anime - any episode that features an "old" Pokémon, they can take the episode to explore these new traits rather than rehashing old ideas. Not only that, even in game terms, it may open up opportunities for older Legendaries to have different roles, whether it be in main games or side games. For example, if new information comes out more directly tying one Legendary to another that wasn't previously linked, then it opens that out more for in-game development. I guess, a better example would be, like if Gen 6 introduced a new Legendary that was related to an older one, giving the old and new a prominent role in the new game so that they can have a bit of a balance between the old and new maybe.