So all but 1 Reserve card are used only for their on-call ability. On play, some of those give +1 action, some are cantrips, and 1 is a Copper... but none of them are things you would want to play; you play them to get them on your mat. And then there's Wine Merchant, which you only play for the on-play ability, it has no call ability in fact, just a way to discard it from the mat.
I'm just curious if it's a good idea to have a card that you both want to play, and want to call. Something that gives a medium-strength benefit on-play (more exciting than terminal silver, but not too strong), and also has medium-strength benefit when you call it. I'm assuming that the main reason nothing like this exists is that the concept of reserve cards is supposed to be "cards that wait until you need them," and this idea doesn't really fit into that. But could this be a fun and balanced idea? It would give you more incentive to call the card sooner; so that you can play it again sooner. So sometimes you'd have a tough decision to make; "should I call this now before I reshuffle, or should I wait until it can be called at a better time?"