Learn Danish: Tja, det kan godt være.

Here, godt is a modal particle used to soften the statement. Together with kan, the phrase kan godt translates to "might well" or "may." It changes a factual "it can be" into a more tentative "that might be the case."

What exactly does tja convey here compared to other conversational fillers like nå?

Tja specifically expresses hesitation, mild doubt, or reluctant agreement, much like drawing out "weeeell..." in English. In contrast, nå is usually used for surprise, realization, or transitioning to a new topic.

Can I just say det kan være without godt?

You can, but it sounds much more abrupt and less natural for this specific expression. Adding godt makes the phrase idiomatic for "that may be" or "perhaps," which perfectly matches the hesitant tone set by tja.