It would be useful if we add an extra "Suite Group", or "Suite Categories" level in the SoapUI test case hierarchy for organizing test suites. This would be a parent of Test Suite. This way you could collapse.expand categories of test suites. This should be an optional level, so that those who just want test suites wouldn't need to have the extra layer of categories.
Reasoning for it's usefulness:
Currently, in each project, we organize tests by test suite, and test case. There is no broader category than test suite. In a situation where you have multiple APIs that while might mostly be independent, but that do also have interaction with each other, it would be handy to have them all be in a single project.
However, if each API itself needs several test suites for testing it, then having multiple APIs in a single project isn't great for organizing your test suites, since they're all in a flat structure at the test suite level. That's one reason for keeping the APIs in different projects, so that you can expand only the test suites for the given API, without seeing a bit list of all the other API's test suites.
The most common usage for Suite Groups/Categories would likely be to organize test suites by each API in the project, but their would certainly be other categories that people would come up with.