I think it might become too common of a drop for key limes. You could amass hundreds of keys that way because key limes are easy to grow. I think they meant for keys to be a special reward, kinda rare. That's why I suggested Ultra key limes.
I would make ultra key limes require more attention than regular limes.
Maybe regular key limes could drop the house item decor chests at elder.
It would be fun if they created another key class called mystery keys that would open mystery chests only and place them throughout the spiral. Or have them open a door to a solo battle only. I wouldn't mind a few solo bosses.
You don't think it's difficult enough already getting them? I think it would lend more variety to gameplay, and it would give us more of a chance to get rare reagent drops like motes of transport that are required to craft mounts. Currently, the only way I know to get a mote of transport is through one of the exalted one-shot dungeons. Which cost crowns, of course.