Hmmm. Thinking to myself here on what areas could be modded ... The plants themselves, plus the seeds, shouldn't be touched. The graphics are lovely already, and any changes would only confuse things.
The Main screen - perhaps some changes to the design of the pots? I imagine the insects would be a difficult job to alter. Backgrounds to the snapshots can be changed easily.
The seeds screen - possibly some changes to the colours of the seed trays? The actual layout couldn't really be altered.
The Collection screen - maybe a different background?
The Nursery screen - changes to the background? Visitor's clothes? Some of the bought garden "furniture"?

It would have to be a very good mod indeed to improve in any way on the artwork already supplied!