koto is a term used to refer to a idea, action, event, etc.
"sono koto desu" - that thing - reference to "that thing we were talking about" = idea
mono is used when the term referred to is usually tangible noun
"sono mono desu" - that thing - reference to pointing to store clerk "i would like that thing"
koto ga hoshii would not be a natural way of speaking, because technically you cannot "want an idea/action/event"
your meaning would probably get across but it's simply the wrong way to use in that form of sentence
watashi wa nanika taberu mono ga hoshii (i want something to eat) is correct
koto ga hoshii is wrong because koto is abstract, refers to the act of eating, whereas mono is concrete ( something ...in this case food )
tabemono ga hoshii (i want some food)
taberu koto ga suki (i love to eat)
hope this helps
i'm not a ntaive
so you may want to wait for a native Japanese to confirm the answer