They need to have legendary cats of ghost, dark, and psychic
I think they could get mixed up horribly easy with the Eeveelutions if they don't make them all bulky and fat, which would destroy the whole idea about cats being agile and slender~
Gen IV had the most legendaries of any game, which would likely mean that Gen V is going to have more of them.
Gen 1 had 5 (with Mew), gen 2 had 6, gen 3 had 10, and Gen 4 had 14 - which means that Gen5 is probably going to have something between 14 and 18.
So, let's look at it this way.
Legendary trio: all games.
Special cute mini event Pokémon: all games.
Evil Legendary: gen I (Mewtwo), III (Deoxys) and IV (Darkrai).
---yes, Deoxys wasn't THAT evil, but it has stuff in common with the other two---
Main two legendaries: Gen 2, 3, 4
Third legendary: Gen 3 (Rayquaza), 4 (Giratina)
Twins: Gen 3 (Lati@s), Gen 4 (Cresselia and Darkrai, Regigigas and Heatran (?) )
The rest of the legendary groups were Gen4-specific.
So, Gen5 is probably going to have these legendaries:
Legendary trio (as Leafeon stated, ghost-psychic-dark? fighting-psychic-dark?)
Main two legendaries (impossible to speculate on, I mean, look at Dialga and Palkia! They're probably going to have a very detailed body, though.)
Evil legendary (I'm thinking ghost-dark like Spiritomb or Sableye - deformed ghost anyone?)
Third legendary (see main two legendaries)
I'm too tired to write out the other categories, but you get the idea.
Speculate freely from this! *phew*
This is Hana, signing out~