All these answers are tip toeing around the actual answer. "There will be no one to stop us this time." refers to two things. The first thing it refers to is that the rebellion has secret, unknown members all around the Empire. The second thing it refers to is that Vader has no idea that Obi-Wan Kenobi is on Tatooine. The Empire would track a suspected rebel to some planet, or some area on some planet, but someone else on that planet, who was secretly a rebel, would end up helping the individual the Empire was tracking, or, someone within the senate, who is also a member of the rebellion, would secretly help a rebel the Empire was tracking. Meaning, just when the Empire thought they had the rebel cornered, someone would "magically" come out of the woodwork and help this rebel in some way. Just as in the Phantom Menace when they say that The Trade Federation would have no control over Tatooine because it was controlled by the Hutts, the Empire knew that no rebels would be on Tatooine because the rebellion had members all over The Empire, and since the Empire did not control Tatooine, there would be no rebels on Tatooine, also because the Empire did not control Tatooine, Vader also knew no one in the Senate could have any influence over Tatooine. Which is also the reason Vader thought that recovering the plans from Tatooine would be easy, he knew that Leia could not be sending the plans, via escape pod, to another rebel on Tatooine. Given Nahdonnis Praji's line: "Lord Vader, the battle station plans are not aboard this ship, and no transmissions were made..." When Vader said "There will be no one to stop us this time." Meant that because Vader knew the Tantive IV could not have sent any transmission(s) to another rebel who could potentially help, and also because he knew no rebels could be on Tatooine, he thought he knew that it was impossible for anyone to "magically" come out of the woodwork and help the rebellion this time. But he was wrong, even though no rebels were stationed on Tatooine, someone sympathetic to the rebellion - Obi-Wan Kenobi - did live on Tatooine, and did "magically" come out of the woodwork to aid the rebellion.

All these answers are tip toeing around the actual answer.

"There will be no one to stop us this time."

…refers to two things.

The first thing it refers to is that the rebellion has secret, unknown members all around the Empire. The second thing it refers to is that Vader has no idea that Obi-Wan Kenobi is on Tatooine. The Empire would track a suspected rebel to some planet, or some area on some planet, but someone else on that planet, who was secretly a rebel, would end up helping the individual the Empire was tracking, or, someone within the senate, who is also a member of the rebellion, would secretly help a rebel the Empire was tracking.

Meaning, just when the Empire thought they had the rebel cornered, someone would "magically" come out of the woodwork and help this rebel in some way. Just as in the Phantom Menace when they say that The Trade Federation would have no control over Tatooine because it was controlled by the Hutts, the Empire knew that no rebels would be on Tatooine because the rebellion had members all over The Empire, and since the Empire did not control Tatooine, there would be no rebels on Tatooine, also because the Empire did not control Tatooine, Vader also knew no one in the Senate could have any influence over Tatooine.

Which is also the reason Vader thought that recovering the plans from Tatooine would be easy, he knew that Leia could not be sending the plans, via escape pod, to another rebel on Tatooine.

Given Nahdonnis Praji's line:

"Lord Vader, the battle station plans are not aboard this ship, and no transmissions were made..."

When Vader said:

"There will be no one to stop us this time."

Meant that because Vader knew the Tantive IV could not have sent any transmission(s) to another rebel who could potentially help, and also because he knew no rebels could be on Tatooine, he thought he knew that it was impossible for anyone to "magically" come out of the woodwork and help the rebellion this time.

But he was wrong, even though no rebels were stationed on Tatooine, someone sympathetic to the rebellion - Obi-Wan Kenobi - did live on Tatooine, and did "magically" come out of the woodwork to aid the rebellion.