The only real target cards I can think of for Plot Convenience would be: a) Mulligan - store a couple of those for opportune flip and draws or b) resource cards - specifically if you're needing to squirrel away specific resource types for power plays. The downside is the cost of two to have merely a placeholder of cards that you'll still need to pay for later (except for resource cards).

In a multiplayer game this will more consistently allow players to bank cards that may be potentially dead or set up bigger future turns. Another usage of this that was missed in the previous review is: You have Mulligan but don’t want to lose one good card, bank the one good card by attaching it to Plot Convenience, then use Mulligan to draw a new hand. If you find ways to use your hand without actually playing a card you can cheat quite a bit of value out of your Mulligans beyond just the hand modification value (see other pool upgrades).