My last real involvement with d20Modern was with Dawning Star, pretty much for reasons stated above. Actions Points are weak, Talents are bland and uneven, characters start with two few feats (the "fighters" often have to burn a feat for Personal Firearms Proficiency at 1st level!), and the classes don't provide clear distinctions. moving away from minutae (Pathfinderizing the skill list, and so forth) the big design priorities for me would be:
- Either reworking the base classes or jettisoning them for general but more archetypally focused ones (like Soldier could be a base class). Adventure! and Spycraft sort of show the way here if you wanted to move away from the basic 6
- Each Occupation and each base class should grant one more feat, that way you could play a US Marine or a ninja at 1st level. The typical D&D class gets the equivalent of about six starting feats, compared to about two to three for a d20 modern character.
- Consolidate less-used firearms options into fewer feats, or open them up as general combat options with feats granting Improved versions. Fantasy Craft and Spycraft have some good examples, as well as the Tactical Feats from Complete Warrior as a template.
- Designate an opponent tier below Ordinary that gets minimum hp, gets the non-elite array, and is limited in other respects (can't normally critically hit would be a good start)- D&D 4e and Fantasy Craft show the way here.
- d20 already has a defense bonus built into classes, but there should be some feats to beaf up unarmored AC.
- Action Point reform - you could probably borrow the version from the Pathfinder APG. At least, they should be potent, renewable, and do more than fuel anemic class abilities and offer uninspiring bonuses on rolls.