First of all, we don't know that ID is made of Seastone. At least, I'm pretty damn sure it was never said.
Second of all, obviously, if he comes back into the main story, then yes, he factors into the grand scheme of things. Why else would he return? He's certainly not popular enough to merit fanservice.
Actually, by "full of seastone" I meant that the building contained a lot of seastone, but not that it was made of seastone.
About the main story, why would he return? I don't get your meaning. He already lost to Luffy and lost his title and Pluton. So there's no point to him challenging them again. I don't see how he'd fit into this current War/Whitebeard storyline, he'd be lost amongst the 11 or more other people as strong as him (shichibukai and admirals and Whitebeard) It'd be selfish to make a new storyline for Croc, I'd rather see the other shichibukai and dozens of other characters get their turn. Croc already had a huge arc, as did Moria.
I'm not convinced that this would be good storytelling.
You seem to hold things in impossibly high regard based on how they're hyped. Impel Down may be this super-prison, but even Alcatraz was escaped from, to use a real-life analogy.
if the great and invincible Shichibukai could have two of its members defeated, then it's safe to assume someone can, and perhaps will, escape Impel Down. And it will probably be a huge deal, to boot.
Enough about hype. The fact that Crocodile was so strong was to meet the hype of the shichibukai, and the man who we waited three or so arcs to meet. It's not hype as much as it is "build up." Croc was built up to be awesome, and when he fought he delivered by nearly killing our undefeated hero. Impel Down was built to be a super prison during the entire Enies Lobby climax, so why should it not deliver?
No one suggested ways to escape it, and there were some smart people at Enies Lobby. The CP9 knew more about the WG then we did. Robin, even in her depressed state, is one of the smarter people in the series and she treated Impel Down like death, like the point of no return.
Why would Croc be able to escape it? He's never shown that kind of ability.
He'd be defenseless. His powers would be gone from the seastone cuffs he'd undoubtedly wear. He'd have to negotiate his way out and he's never shown that much charisma. He only lied to people that didn't even know him. Here everyone knows him. He gained power gathering it secretly using his free reign as a shichibukai. Now in Impel Down he doesn't have that freedom. He can't run a casino. He won't be fighting pirates. He cant' fund agents, and he doesn't have Robin to act as a general to those men. In Impel Down he has far less to work with, and has to accomplish something perhaps more difficult than conquering Alabasta.
For someone missing alot of his resources, he seems pretty confident last time we saw him. It's not farfetched to assume he has a backup plan. Hell, he's known for having tons of backup plans.
He never planned to lose in Alabasta, so he would have had to come up with this plan in prison. In fact, he never planed to escape! His lesser agents broke them out on their own. He actually would have had to come up with this plan as the marines came to rearrest him. That seems like a stretch, since his last plan took years to accomplish. And what is this plan? What could he gain in this high level prison?
Making a crew of prisoners is lame. They'd have to be weaker than him, making is as strong as his Baroque works crew. Except this time he'd be an enemy of the world. He'd risk fighting the other shichibukai, admirials, Luffy, and thousands of rookie pirates crews again. Would he really want that?
Finding "something" is vague. Why would there be some valuable item in Impel Down, and why wasn't this mentioned? If it's not better than Pluton than he'd still be underpowered to fight the world.
I think that the end of the cover story, and Crocodile in Impel Down, was a way to write some powerful villains out of the story without killing them. Impel Down is a "point of of no return." You mention Enel becoming a god, but it's more than that. He's also trapped on the moon, with his Maxim gone. Though true to his character he is the God of a new land. More importantly, on the moon he can't use his abilities to hurt anyone again, which like many One Piece villains is a big issue.
Buggy is still in chase.
Kuro is trapped as a pirate. If he attacks anyone, than the marines will chase him again.
Arlong is in prison.
Wapol is a rich and reformed man.
The CP9 are currently the heros of a small town instead of killers.
These cover stories write the villains out of the story without killing them. I think that's what the Baroque Works cover story was. The end of Crocodile. Unlike Wapol and Hatchi he'd never change, he'd be a man that killed on a whim. That smile wasn't a plan, but his character. Even in this state of defeat, he's still confident and arrogant as ever. He's not afraid even if he's going to jail (notice Mr. 3's panic). He'll probably do okay for himself in jail, but why would he escape?