This is a good question.
Hypothetically, lets assume a Half-Orc Pure Barbarian Frenzied Berserker with a grand total of 9 feat slots to fill. What Feats are going to be important them?
THF
iTHF
gTHF
Power Attack
Cleave
Great Cleave
Overwhelming Critical
Barbarian Active Past Life (counts as Toughness Feat)
Improved Critical
Completionist
That looks like 10 feats ... Personally, I would drop gTHF. But, I don't know many Completionists that chose Barbarian for their final life.
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Let's assume a Halfling Pure Rogue Assassin build next. Still, only 9 feat slots. I am assuming an end weapon choice of Celestia instead of Khopeshes. I would assume important feats would be:
TWF
iTWF
gTWF
Precision
Barbarian Active Past Life (counts as Toughness Feat)
Improved Critical
Dodge / Mobility / Spring Attack (for the Dodge +%)
Power Attack / Cleave / Great Cleave / Overwhelming Critical
Improved Sneak Attack
Completionist
That's up to 15 feats. I am thinking what would probably be dropped would be Mobility, Spring Attack, Power Attack, Cleave, Great Cleave, and Overwhelming Critical.
Personally, in both situations, I would rather Completionist be just auto-granted.