Baseline Grid / Set Up Guides
The ability to set up grid systems for layouts like in InDesign. (Baseline Grid etc.)
Derek S. commented
+1 It would be great to be able to grid layouts like InDesign, especially to lay out type. InDesign is great for this, but sometimes its tendency towards print design is not necessary. Also, having the more advanced vector graphics editing in Illustrator for these types of layouts would be great, without having to import or link these vectors into InDesign.