Heh, it's not an easy answer, but let's talk about the AI. With (at least) four different alien races, Bungie needs to design each to be visably distinctive in the way that they fight and the way in which they are thought. And it needs to be way more complex than "Hurr, the Cabal hide behind their shields and have tons of armor and tons of firepower and are slow, while the Fallen are quick agile and cunning". The moment the races start to blend together is a moment where the game becomes less interesting. And yes, I feel the AI needs to scale quite a bit, simply making our enemies tougher and make their weapons hurt more will make the game more difficult, but it won't make the game more challenging or interesting. More important that simply creating a huge number of weapons to find and collect, Bungie needs to design their sandbox to accomidate a complex and interesting metagame. There should be numerous ways to build your character, avoiding putting dominant strategies in the game as possible is a must, otherwise you'll see all the Titans built one way and all the Warlocks built another. It should feel like you have a couple dozen different ways to take your character and that they are viable to some degree. New content is always nice as well for extending the longevity of the game, as is simply giving players a large number of things to do. It will be like Skyrim, except the core gameplay doesn't make me want to bludgeon someone with a brick. Should be fun if they can pull it off, and if they falter in the process, I will be more than happy to encourage them to succeed with the sequel with constructive criticism.