If Sabo got captured, he would be executed immediately without all the fuss that Ace got. Ace was only executed publicly because he was Rogers son, and the Marines wanted that to be known. Sabo's dad is just a random No Name noble from the east blue.
Also, Blackbeard has no reason to capture Sabo. He captured Ace to convince the marines to make him a Shichibukai, and now he's already a Yonkou.
To take his DF? To lure Dragon into a war so that they can take his DF, or to just kill him, the man known to be this huge background figure and Blackbeard kills him gaining even more infamy... I didn't mention the marines at all, this would be a war between Blackbeard and Dragon, to which Luffy could turn up and do what he did with Whitebeard, not acknowledge Dragon for anyone other than his equal and declare how he'd get Sabo back himself xD
Like I said, this would be very annoying of Oda to do, but not at all impossible.
Sabo isn't admiral level.
Kaido is the strongest character alive, so Dragon isn't above Roger unless you think that Kaido>Roger.
This title has been bugging me for a while, Whitebeard has strongest man, rightfully so, Kaido now has strongest creature, is that either the hint that he has a zoan devil fruit and blah blah blah or is it just his overwhelming resilience to the fact that he can't die, making him the
strongest in endurance terms, but strength in fighting capabilities not the strongest... I hope it's the latter personally..