Background- These are a few ACFs I've homebrewed in the past for short campaigns and one-shots. Enjoy!
A Special Suprise
Sometimes, a Beguiler wants to go into combat and leave a special gift to the people they oppose.
1 1st level spell slot, Suprise Casting, Cloaked Casting
The Beguiler's HD becomes d8. Also, at 2nd level and every 4 levels after, the Beguiler gains +1d6 Sneak Attack damage, as a Rogue. At 4th level, the beguiler may choose to sacrifice a number of Sneak Attack dice to cast a spell he knows as part of the attack.
Animal of the Prime
Sometimes, a druid never learns just how to turn into an elemental, instead turning into elemental versions of creatures.
At 16th level, you may apply the Fire, Water, Air, or Earth Elemental creature template to a creature you transform into through Wildshape 1/day.
At 18th level, you may apply the template 2/day, and at 20th, 3/day. At 20th level, you may apply the template to a Huge creature you Wildshape into.
Power at the Pores
Some arcane spellcasters burst with magical energy that they cannot help doing it all the time.
Any arcane spellcasting class that can cast cantrips
Whenever spellcasting is gained
One level 0 spell slot
The spellcaster gains Magic Man
as a bonus feat.
Some fighters learn maneuvers. Others learn a Signature Move.
Any level that a fighter learns a bonus feat
The fighter must have learned the maneuver by way of the Martial Study feat, selected as a bonus feat. Also, the fighter cannot have any levels in a maneuver-using class.
The fighter, given time, can become more familiar with a certain technique and how to use it. The fighter selects one maneuver he knows, which must have been learned through the Martial Study feat. The selection of this feat must have been as a fighter bonus feat. When this class feature is used, the fighter learns how to use the maneuver more often. Upon selecting this class feature, the fighter may use the maneuver 2/encounter. This class feature may be selected up to three times for all maneuvers learned this way, each time adding an additional use per encounter to the Fighter's use of the maneuver.
Not all monks learn how to use flurries of blows, instead taking advantage of an enemy's unwareness of an attack.
Flurry of Blows
At 1st level, 5th level, and every five levels after, the Monk may gain +1d6 Sneak Attack damage, as the Rogue class feature. The Monk also learns how to repeatedly attack an unaware foe. After making a Sneak Attack, the Monk may make another Sneak Attack as though the target still qualified to be Sneak Attacked, although at one less die of additional damage. This may be used to allow an additional Sneak Attack even if the bonus die would equal +0d6, although such a Sneak Attack can only be made once. +0d6 Sneak Attacks are made with a -3 penalty on the attack roll. Also, additional attacks may only be made if the previous attack was successful.
Please PEACH, and feel free to make recommendations for more ACFs, as I may just make them when I have the time.