I'm just thinking about the design space for this contest, to get my own creative juices flowing and hopefully this inspires other people as well.
Obviously the card has to interact with the heirloom to be interesting so it has to both:
(1) do something that cares about cost
(2) do that after a treasure is played.
All current submissions do this.
Spectacle gives a buy(1), and is a night (2) that can bring you back to Action/Buy phase.
Port Clerk gives a buy(1) which comes into affect for the (2) buy phase. Port Clerk also cares about unique treasures
Road gains a card per cost (1) which comes into affect during cleanup (2 -- happens after treasures are played)
ideas for option 1:
Gain a card costing up to X, +Buy, Trash for benefit*, Remodel/Upgrade*, Discard for Benefit*, Attack: other players Trash/Discard a card costing at least X (to be sensible probably have a max as well), do something if the cost of a revealed card is exactly X.
* These all can get worse with cost-reduction, of course.
ideas for option 2:
Make it a Treasure Card, make it a Night Card, make it a Reserve Card that can be called later, make it an Action that has a trigger later (like Road's on-discard-from-play), make it an Action card that gives out a State that does something at end of Buy Phase, make it a card with the ability to play Treasure cards not during the Buy Phase