Okay, I stumbled across this during a bout of insomnia and am inspired to be verbose.
I would love to see the data generate a map for each plant. Being able to see the trend of where a plant likes to grow at a quick glance would be wonderful.
For example, when someone chooses to "Rate a Plant" that data entry would be represented by a colored dot, such as:
Green dot= Performs well and, survives both heat of summer and cold of winter
(if a plant was a true annual, it would only need to perform well in the appropriate season)
Yellow dot= Performs so-so, or survives summer or winter intermittently
Red dot= Performs poorly
and also..
Purple dot= Performs well but requires special handling (such as over wintering indoors) Purple is, of course, because these plants are regal but require some pampering like royalty.
Each basic rating could have an option to choose a short detail that would show up when a map user hovered over a dot. For example: Red dot could have: died in summer heat, died in winter cold, survived but performed poorly.
Purple dot could have: give extra winter protection/mulch well, must overwinter inside, used as winter annual only, etc.
Being able to zoom to state level with county lines depicted would be helpful.
I'm not sure if you could get permission to use them, but, if so, you could even overlay the date dots onto Köppen–Geiger climate classification maps so you could see the general trend of which climates the plants preferred. Since most of the data would, most likely, be from the USA, that might be helpful to gardeners in other countries since they could go by the climate classifications.
Being able to map where a plant was native would be really nice, too, but that would require a lot of data entry so probably wouldn't be feasible unless you had a way to import that info (or a lot of volunteers.)
Okay, enough rambling from me for now...
I know that whatever you end up doing will be wonderful, as usual!
