Perhaps, if every single person responding to you is missing your point or blurring your lines… you are not explaining yourself very well.
Your main criticism at current is that the villains are jokes lacking credibility, correct? So... how would an arc focusing on them, where they accomplish nothing, rectify that?
For if they stalemate or lose to an opponent, or allow them to escape, then they lose credibility. So Marines must capture and imprison someone important, (except they've done that repeatedly. and also managed to lose a ton of prisoners. SO simply capturing someone big must not provide credibility. We've seen the Buster Call obliterate two islands now, including an entire swath of innocents, so that must not add credibility. And we've seen how they torture prisoners in Impel Down, so that must not add credibility)
While Blackbeard would have to...?
Aikainu has already killed tons of people, including all of Ohara, Ace, and contributing to Whitebeard. Blackbeard I gave a laundry list of his accomplishments, including defeating and killing some of the strongest men in the world, (in a series where no one manages to kill anyone) destroying a small country and completely obliterating an island, all of which you dismissed as inconsequential.
So if the marines capturing pirates (and even killing a named character) isn't enough, and Blackbeard killing pirates and destroying landmasses isn't enough... what exactly do you feel needs to be shown to make them more threatening?
Defeat a lot of fodder? Can't be that, since Luffy can knock out 50,000 guys with a yell now. Defeat another powerful person, aside from Ace and Whitebeard? Can't be that, since you already find that insufficient and that was the strongest man in the world.
Show the plight of Smoker and Kuzan working as underdogs who doesn't like the orders coming from on high? We've already gotten that too.
I'm not sure what that could amount to, aside from the death of Shanks, which IS coming, but no, won't be a DBZ fight. Oda's going to save showing off the evil crew's powers for the Strawhats to discover.
You still haven't answered the question of what such an arc focusing on tertiary characters would actually add to the story that isn't already being provided. WHAT would a divergence that loses the focal point characters add to the story in this case?
So please. Make it clear since we obviously lack reading comprehension.
In one sentence, what do you want to see in a side arc focusing on the marines or Blackbeard, that hasn't already been shown or mentioned that would actually escalate their threat level, but that doesn't involve the harassment of innocents, winning a fight, capture or death?