There are some low expectations. Take Hanifin for example - Even he scored 22 points and in terms of talent these two aren't even close. Aron Ekblad's rookie season indeed should be seen as the minimum expectation. Of course, it doesn't mean that the season is a failure if he doesn't achieve that but I don't really see why he wouldn't.
He's an offensive defenseman first and foremost, I fully expect the point scoring to be there. I definitely don't see the possibility of his season being " sub 20 points but solid defensively" at all. If he's scoring sub 20 points, I expect it to be due to him not being ready for the league, not due to him completely prioritizing defense - That's not his game and I don't think that'd be natural.