Indeed, there’s nothing new under the sun.
What was the difference between these two offerings and why was one accepted and the other rejected? Offerings to God are theological statements.
There's simply what the account states:
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
Cain: of whatever fruit, nothing special.
Abel: the best of the best of the flock.
Cain merely crossed off another task from his to-do list.
Abel really wanted to give a special gift, and put some thought into selecting it.
Then Cain was offended that he got no respect. Abel made him look bad just on general principle.
The “Third Category” concept is an attempt to lay a ploy on Yehova.
Not bad enough to go to the lake of fire?
Sorry, no cigar! Obedience is the requirement for salvation.