Learn Finnish: Luullakseni tästä sukutarinasta tulee vielä ongelma. - I suppose this family story will still become a problem.

It comes from the verb luulla (to think or suppose). It is formed using the 1st infinitive (luulla), adding the translative case ending (which takes the form -kse here), and finishing with a possessive suffix, like the first-person singular -ni. This structure translates roughly to "as far as I think" or "I suppose" and acts as a sentence adverb to express your viewpoint.

Why is "this family story" in the elative case (tästä sukutarinasta)?

When expressing that something "becomes" something else, Finnish uses the verb tulla with the original thing in the elative case (-sta or -stä). Literally, the sentence is saying that a problem comes out of this family story.

Why is the result of the change, ongelma, in the nominative case instead of the partitive?

In the jostakin tulee jotakin (something becomes something) construction, the final result is always in the nominative case. So, while the source is elative (sukutarinasta), the outcome (ongelma) takes the nominative.

What nuance does the word vielä add to this sentence?

While vielä usually means "still" or "yet", in this context it carries a tone of "eventually" or "inevitably". It implies that the problem hasn't fully materialized yet, but the speaker strongly expects that it will happen in the future.