You buy stuff when you need it.
Ok?
Just in case you haven't understood this yet:
Saying "I have an oscilloscope, what tool should I buy next" is, perhaps, the wrong question to ask. You should be thinking about which
projects to work on next. As you're working on/preparing for those projects, you'll find yourself thinking:
-- "I need to input a signal into this circuit to test it". OK then, buy a signal gen.
-- "I can't power this circuit!". OK then, buy a PSU.
-- "I can't analyse the timing issues on my PCIe circuit!". OK then, buy a 40 GHz $100,000 oscilloscope, then.
For example, I like working on a lot of digital stuff, so I've never had a signal genenerator. On the other hand, I program Atmel micros a lot, so I splurged on an Atmel Programmer/Debugger. My neighbour might be the opposite, he might well be far better off with a signal gen than a debugger.
The point is, no-one can tell you what tools
you should have until you find out what's holding you back from doing the projects that interest and excite
you. Wait until you're frustrated by not having something, and then buy it.