How would the card be mapped onto the filter that should apply? When cross-filtering from a chart, it is the axis field that is used as the filter. But cards don't have an axis. (Yet, as Small Multiples is planned for the new card). Let's suppose you have a measure "% of sales that were in France". Presumably, you would want clicking on the card to filter the page to France. How is the measure turned into a page filter? I don't think it can be done in a systematic way. I would suggest the following: If the card has an filter set in the filter pane, offer that as the page cross filter. Additionally, offer a "cross-filter" group in the filter pane with a card selected. This would allow selecting "none", "card filter" (where set) or setting a whole new cross-filter.
