In my opinion, boosts should be focused on making building relationships easier - I think that after having achieved all the NG+ bonuses we should be able to start with, collectively:
- +25 People (to be able to pass Zarad's date)
- +25 Intelligence (to be able to pass Lyon's date)
- potentially +25 Courage (whether for Jarrod reasons, Jasper romance reasons, or passing the self-intros)
I also love whoever on Tumblr suggested a +50 gold bonus for picking out all the best presents for people, the lack of money can really be a killer sometimes (please, my Scholar and Countess are dying...well just the Scholar but)
Other achievement bonus ideas I wrote down earlier:
"Marvelously Misjudged" achievement: +5 charisma
"A Dangerous Secret" achievement: +5 cunning
"Holey Investion [sic]" achievement: +5 warfare
"The Lady, The Legend" achievement: +5 charm
"Birthright" achievement: +5 courage
"Water Wears Stone" achievement: +5 poise
"No Hallway Unlurked" achievement: +5 street smarts
Corval-Hise Legendary Peace: +10 Cunning
Wellin-Skalt Legendary Peace: +10 Practical
Jiyel-Arland Legendary Peace: +10 History
Revaire-Rebels Legendary Peace: +10 Politics
Also, this seems like a long shot, but I thought I'd throw it out there:
Katiya's Legacy/Golden Ending: +5 to every skill
100% Bonus/seeing all content in the game: +5 to every knowledge (minus secrets)