Exactly how would an item change an ability? As of right now abilities rely on their tier levels to augment their behavior, so would this proposed item add an additional fourth tier? To me that sounds fun, but it ultimately adds a lot of work that could be added somewhere else, as the developers would need to add a fourth tier to one more abilities per hero that may see little if any use, because it requires an item to be purchased.
And now that we are on the topic of Aghanim's Scepter. As fun and iconic as it is in Dota 2, I think a lot of people agree that it hurts build diversity, as the developers from a balance stand point want it to be equally good on all heroes, which means it also ends up as a core choice for many heroes. That ultimately means that there's a lot of work put into an item choice that ultimately forces players to buy it in every game.
I like Aghanim's Scepter as an item that radically changes abilities, but I also dislike if for the above reasons. And I believe the developers nailed ability point spending in this game, as they each behave as their little Aghanim Scepters' for each ability. You are not putting points into them, because it makes them scale. You are putting points into them, because the tiers expand on their behavior.