Abberation - As is, plus Ooze and Plant. Ooze and Plant help fill out the fodder role, as well as extending the level spread and increased rarity of Abberation.
Beast - Include Animals, Magical Beasts, and Vermin. Extremely common enemies found at all levels, but lacking many notable bosses or difficulty factor. This category still needs work to be a contender.
Construct - As is, constructs are not quite as common as other groups, but the extra toughness factor helps them stand out as a top pick.
Elemental - As is, plus Mephits. Found at all levels, Common and Tough, but lacking notable bosses (plenty of easy bosses).
Extraplanars - Tieflings, Mephits, Djinni, Tharaak Hounds, anything and everything not covered by either elemental or evil outsider. Good level spread, very common, but lacking in tougher enemies and bosses. (I realize this name is rather generic, and could be confusing if people thought it included Elementals and Evil Outsiders)
Giant - As is, Giants fulfill all criteria nicely, and are already a top pick. Only concern is rarity at higher levels.
Humanoid - Humans, Elves, Dwarves, Halflings, Gnomes. Very common enemies found at all levels, but not very tough, and very few notable bosses. Still a bit unbalanced, but inclusion of Drow and Duergar should help.
Humanoid, Monsterous - As is, but include Orcs, Goblinoids, and Gnolls. Mostly fodder, with a few tough enemies. Found at all levels, though it thins higher up. A few notable bosses (medusa, mostly) but light in that area.
Outsider, Evil - As is. Lacking in low level content, but more than made up for by being common, tough, and having a multitude of bosses and raid bosses. Possibly include Tieflings, unless this would make EO "too good".
Reptilian - Kobolds, Trogs, Medusa and Dragons. This category also needs some help, as it is still not a real contender. This category could perhaps be divided and merged with other groups (dragons into Beast, rest into Monsterous Humanoid?).
Undead - As is. As stated earlier, meets all criteria.