I am reminded, for no particular reason, that the American English idiom to fill [someone’s] shoes is entirely unconnected to the British English idiom to fill [someone’s] boots.
The US idiom is about replaceability—when a person is replaced in an institution, the institution must find someone to fill their shoes. This is true whether or not they are dead men’s shoes—which are opportunities created by someone dying, or more pleasantly, moving on somewhere else, and which is generally, I think, negative in tone. If, for instance, a Lieutenant Governor were to assume the Governor’s office on the Governor’s death, one would disparagingly refer to the new Guv as wearing dead man’s shoes. If an admired Governor chooses not to run for re-election, however, the candidates would have big shoes to fill.
The British idiom (and I am a pathetic anglophile but not to the manner born so I may be getting it wrong) is about surfeit. If I set out food and urge you to fill your boots, I am urging you to dig in. I think I’ve seen it mostly used about greed, though, when one or another corporation (or politician) is said to be filling their boots, often while crying poor.
I don’t know if there are other kinds of footwear that can be idiomatically filled… Filling my sandals would be impractical, and thus make a fine idiom for other sorts of impracticality, but as far as I know the idiom doesn’t exist. I might fill my loafers, so as not to be light in them, but that seems both clunky and, you know, awful. Filling one’s socks doesn’t sound good at all. I don’t even know what filling your slippers would entail. Filling your sneakers? Filling a mule? Perhaps shoes and boots are enough.