When I got my fork bags a few years ago, I had the same thoughts as you: sleeping bag one side, tent the other. Found neither would fit. So I used them for clothes: riding clothes one side, civvies the other, and a few soft things like towel to fill the gaps. I don't think hard objects like cooking kit would be good candidates, though something smaller like toolkit might be good. But you'll have to try and see! I now have a smaller sleeping bag and tent, but so far this has meant that I've only needed rear panniers...