The thing with assignments such as these is not "what can I think of". The important thing is "what can I find by reading things".

Reading one thing will give you ideas for other things to read. If, when reading about Locke, you see "Like Hobbes, Locke assumed that (...)", perhaps you should have a look at that Hobbes person. Then you'll read "In Leviathan, Hobbes set out his doctrine of (...)" - perhaps it's time to dig deeper in that 'Leviathan' thing. Turns out Leviathan is all about the "structure of society and legitimate government, and is regarded as one of the earliest and most influential examples of [X]". Sounds like [X] is important, let's check it out. You'll find there were plenty of philosphers writing about [X], and that there were competing ideas critical of [X]. There's plenty to write about!

In just 5 minutes of work I've found enough material to write a huge paper - and that's just using one starting point going down one track; perhaps following different leads I would find something else interesting, and I'm sure starting with Calvin instead of Locke would lead to something else to write about.

Now I got all of this from Wikipedia. Wikipedia is great for getting an idea of what's important and what's related to what, but it's not that great as a source. Luckily Wikipedia always has external sources, which you can look up. Those external sources probably list other sources, which you can look up. And so on and so on. If you're writing a high school paper, this approach will probably be fine.

If you're writing at the college level or above, you'll want to be more careful in the sources you use, and you'll want to use proper bibliographical databases.