A "speculative special ability" is one that hasn't yet been written up, and may not yet have been named - such as the aforementioned "something fairly radical in the way of a special ability". They're difficult to price or balance, just as "vaporware" is difficult to install.
As for "Slayer", some likely variants here include one or more of the following:
*Able to automatically bypass the DR of a particular type of target, such as Undead, Constructs, or Evil Outsiders.
*Being especially resistant to the attacks of a particular type of target, either via a saving throw bonus, a minor immunity, a continious protective spell effect, or to a specialized Damage Reduction (Say 4/Not being the favored target type at L5, increasing to 8/not being the favored target type at L10 and 10/not being the favored target type at L18, to go with Tyonisius, above).
*Having attribute bonuses that only activate when in battle against a particular type of opponent.
*Being able to sense when the appropriate target type is nearby.
For a minor magical dabbler, perhaps from among
*The ability to use Wands and Staves. Perhaps being able to increase their effective caster level, perhaps by one-third their own level.
*Minor Spellcasting - say L1: Select 3xL0, each usable once per day (may select the same one more tha one, choose from any list, may swap out one spell per level gained). L4: 4/2. L7: 5/3. L10: 5/3/1. L13: 6/4/2. L16: 7/5/3, and L19+: 7/5/3/1 (thus getting a single L3 spell). Characters who limit themselves to a particular list get bonus spells based on the relevant attribute, so minor acolytes or druidic initiates can use wisdom, arcane casters could pick charisma or intelligence, and so on. Spell books are not needed.
*Access to some low-powered magical system, such as Runecraft or Witchcraft (Eclipse again)
For a "Winter Huntsman" how about
*The ability to move on ice and snow without slipping or leaving tracks at L1
*A minor (5-10 point) resistance to cold at L5
*The ability to create weapons and armor made of ice as needed at L10 (these melt moments after leaving the creators person. If the user is attacked by fire, they;re destroyed, but take 20 points off the damage while melting).
*At L18 maybe immunity to cold, or having all weapons treated as +2 Frost weapons or some such?
The possibilities are fairly endless of course, but half a dozen flavorful ones will provide the players with a quick and easy template for coming up with more. Personally, I'd say that most of these are worth anywhere from 1-3 Feats, depending on what options are taken.