If your tractor has a mower deck you'll definitely want to keep a third hand in your toolbox for deck installation...
Really, it's hard to answer your question without knowing what you intend to do. If you're talking about setting up shop as a mechanic I'm sure there are several specialty tools you would need. If you're just interested in general maintenance and repair of your own machine, your existing toolbox should probably be just fine (as long as you have metric tools).
A few things that seem to get a lot of use with me:
1) A micrometer - seems I'm always needing a precise measurement of some bolt, seal, ring, shaft, etc.
2) Really long needle nose pliers
3) Snap ring pliers - Kubota loves snap rings
4) 3-jaw puller - saves a lot of prying and beating when you are removing things like pulleys.