Gen 3:15 is a puzzling passage but it’s usually taken to infer that there would come a ‘man’ who would take it upon Himself to crush the authority and rule of satan symbolised by the serpent (‘he shall bruise your head’) but, in the process, would also be physically injured (‘you shall bruise his heel’). Certainly, in retrospect, it reads this way and it’s possible to see in the verse the first declaration that the Christ was promised to sort out the sin just committed by both Adam and Eve.
But, to the serpent (and it must be remembered that this prophetic word was spoken to the serpent and not to either Adam or Eve - though, if anyone of the two would have recourse to listen to it and take it as a promise concerning their offspring then it should have been Eve for mention is made of ‘her offspring’ not ‘his offspring’ which would have referred to Adam) it would mean little more, initially, that, far from satan achieving unchallenged rule and supremacy over mankind throughout subsequent generations, he would find that he would be at continual war with them (the word ‘enmity’ according to Genwen [page80] ‘...implies repeated attacks by both sides to injure the other. It declares lifelong mutual hostility between mankind and the serpent race’), both camps inflicting injuries upon one another. Ultimately, though, the ‘head’ of the serpent, the symbol of rule and authority, supremacy and dominion, would be broken, and man, by implication, would stand triumphant.
Realistically, this had to be achieved in Christ - but that’s to read far too much in to the verse if we’re to take it as ‘standing alone’ in context with no other defining passages of Scripture to give it colour and meaning. Prophetic words rarely give us the method that God’s intending to use to bring about His will and purpose and certainly this is the case if we try and understand what all three recipients (the serpent, man and woman) of the message understood by its utterance. For the time being, then, satan (or, perhaps, we should say ‘serpent’ as the personification of evil into one entity labelled ‘satan’ is going too far at this point in the history of mankind when there’s no other defining passage from which to conclude this) realises that, though he’d tried to achieve rule over the Creation (see the subject ‘Creation/The Restoration of Creation’ part 2 point 3), God’s crowning glory, mankind, would be at continual war with him - just as mankind would be in continual rebellion against the Creator.
God, having declared war on the serpent through His creature mankind and declaring that, ultimately, mankind would triumph over the rule of the serpent, had no one else to choose except Adam and Eve to bring Messiah to earth. Only two humans then existed so the offspring had to come through them.
Alternatively, He could have begun again with a new Creation and creature but the same possibility of degeneration and rebellion would have existed in that new world when freewill would have been sown in to the framework of a being. So He chose rather to bring what was perfect (Christ) out of what had become tarnished by sin (mankind and Nature - tainted through subjection to the one who had sinned), to restore both the fallen Creation and the fallen creature.
Between Adam and Abraham there was no specific plan brought into being by God to bring the Messiah to earth - at least, we have none recorded for us in the pages of Genesis. But the ‘line’ of choice (and the first specific promise of God to an individual that they would ‘father’ the ultimate hope for mankind) began with the calling of Abram (the first Jew - subsequently named Abraham) in Ur of the Chaldeans and then, after a break in the journey, Haran (c.2000 BC).