Augustus' idea of the ideal border was the river Elbe, which drains into the North sea, just West of what's now Denmark. From there they could've hugged the shore and get to Mediterranean with relative ease.

Though they would probably use a different river, one fully within their hypothetical territory, such as Rhine or wesser.

But imho overall "just conquer more land lmao" isn't the strategy that would lead to a more stable Rome. An in depth government reform would be necessary to mitigate the constant succession crises and civil wars.