Polish ditinguishes between perfective and imperfective verbs:
jeść [imperfective] - zjeść [perfective]
So this sentence means something like: "I want to have eaten something good".
"Chcę jeść coś dobrego" would mean that "I want to be eating something good [now]" or "I want to eat something good [regularly]"
Often one English verb can translate to 2, 3 (or even more) different verbs in Polish, depending on tense (and additional adverb) used in English phrase. See "Aspect of Verbs in Polish, Verbs of Singular, Multiple and Completed actions"
I think dobrego is an adjective describing the noun coś, which appears to be in the Accusative. Is dobrego in the Genitive, or is it actually Accusative masculine personal or animate? If Genitive, why would that be? If Accusative, what makes it personal or animate as opposed to the expected inanimate (assuming the something you want to eat is not still alive and kicking :-)
"Coś" and "nic" when used in Nominative and Accusative generally connect with adjectives in Genitive.
You eat something nice in UK English ... should not be ranked as a wrong answer.
Does this have to mean 'I want to have eaten something good' or can it also mean 'I want to eat something good' (In the future, just one time and finish it) Thanks! :)
The second one. I don't even understand when would one use the first sentence... "Geez, I wish that soup I have just finished was tasty"?
Does coś have a gender? Dobrego is accusative, genitive masc anim and masc personal. Wiki does not specify a gender for coś but it certainly does not sound like a masc anim or masc personal type of word.
"coś" is neuter http://sgjp.pl/leksemy/#90003/co%C5%9B - more precisely, n2 = 2-nd type of neuter i.e. it takes cardinal numerals, while nouns type n1 take collective numerals https://en.wikipedia.org/wiki/Polish_grammar#Numbers_and_quantifiers