it's sad really. i love many of those you listed (especially bortkiewicz, medtner, and grainger!). actually isn't there a quote floating around somewhere where rachmaninoff even stated he believed medtner to be the most important or best composer alive at his time? i'll try to dig it up if someone else doesn't beat me to it.
bortkiewicz, i don't know why he fell into a period of relative obscurity, he wrote some of the most beautiful piano music i have ever heard (and if you missed either thal's or my agreement with his sentiment that his piano concerto no 1 is one of if not the greatest of romantic piano and orchestra works, you owe it to yourself to give it a listen, his etudes are also astonishingly beautiful).
grainger i think fell victim to his 'out of place' style, he was more in the tonal vein and tradition of composition at a time when there was a lot of buzz and experimentation and fascination with new atonal techniques. there s a really good book out there on him i came across years ago with a section exploring and explaining why he didn't /doesn't get the attention he prolly should.
i would also add kosenko to that list (if you missed his piano concerto yt link i posted in the concerto thread yesterday). i like him a lot. if you want to learn a bit more about him, i uploaded some notes to my scribd feed yesterday which talks about him and his incredible op 19 set of etudes-of which he wrote more than one set ( if you like baroque dance styles, bach, etc. these op 19 are well worth your exploring
https://www.scribd.com/doc/118746155/Viktor-Victor-Stepanovych-Kosenko-Piano-Music-Vol-1-Eleven-Etudes-in-the-Form-of-Old-Dances-Op-19 )
example
this seems to be the most 'popular' (at least amongst those that even know of its existance) of the bunch:
i've also not seen this specifically discussed but this is by far THE LONGEST ETUDE I HAVE EVER SEEN. if there is a longer one, it must be a monster becuase this thing is huge. it is also one of the most varied etudes i have come across with regard to the multitude and variety of technical challenges it presents (see notes i linked to), as most etudes usually have a single or small set of related difficulties you must deal with, this one is all over the place!