A trilogy of books will definitely give more space for goodness.
Players do not need more than a Players' Handbook, and then they can have a full 300 (or hopefully 400) pages of races, classes, feats, powers, essential classes, options, themes and other goodness.
Leave out everything past 10th or maybe 15th level for an Epic/Paragon book. More space for more options, especially since people want this new space-devouring modularity.
Put the magic items back in the Dungeon Master's Guide. No more +X items, thus reducing the need for redundant space-wasting lists of +1 swords, implements, maces, bows and nunchuks. Include henchmen and artifacts.