I'd like a new golden cookie effect: temporarily boosting the number of a given building you are considered to have.
For instance, for the duration of the effect, you are considered to have twice as many Prisms, or three times as many Antimatter Condensers, etc, down to twelve times as many Mines, or thirteen times as many Farms. Cursors and Grandmas would be multiplied by seven, since the achievements for generating cookies from those buildings are the same as for Shipments, which would be multiplied by seven. (Alternately, if this is too powerful, you could just add a flat number: 10 Prisms, 20 Condensers, etc.)
This would affect things like the Synergy upgrades, the Cursor "Fingers" upgrades, and the special Grandmas (if the Grandmas are boosted). You could not, however, sell the extra buildings, since you wouldn't actually have them; you'd just be considered to have them for the purposes of cookie generation. Similarly, the cost of the next building of that type wouldn't go up while the cookie effect was going on.
EDIT: You also wouldn't be considered to have them for the purposes of unlocking upgrades for those buildings or for getting achievements for having a given number of those buildings.