I designed this ability for a Normal type regional Cramorant. It works like follows:

• After being hit by a damaging move, the Pokémon Stockpiles 1 and remembers the type of the received move.
• If the mon receives another attacking move, it Stockpiles again and overwrites the remembered type.
• Spit has the remembered type.
• Swallow grants a resistance to the remembered type until your next move.
• The Stockpile limit is still 3, but it doesn't give Defense and Sp. Def. boosts like when using Stockpile manually.

It's a bit of a gimmicky ability, like Color Change or Forecast, but it might make Swallow viable (I don't think Spit, would make the cut, though)

Would you like to see a mon with this ability? Would you change the way it works somehow?