is it ok to start directly on the new campaign? I really do not have time to do both campaigns. Will I lose anything?
Can I start with a character from scratch directly to northern territory or is it expected to be already leveled up from the previous campaign?
You aren't expected to be already leveled up. It doesn't hurt though, some challenges will simply be too hard until you have enough power.
You will miss out on some extra progress points if you don't do the original campaign.
Also, campaign in the expansion, "Child of Light," will be mean a whole lot less if you don't at least read the campaign in the core game.
The expansions campaign is a straight up continuation of the original campaign, not a separate storyline.