I was thinking about this earlier and I came to two conclusions:
>There shouldn't be TOO many unanswered questions since most of those arise after the (predicted) ending point of the anime. The thing that irks me the most about this series is that there are tons of revelations with explanations few and far between. Even at the current point in the manga, there are still tons of unanswered questions from the beginning of the series as well as those that have piled up all throughout. It seems to be a ways off until any explanations will be given, so for now I'm just gritting my teeth and waiting.
>The anime's end is entirely up in the air. The current pace matches the manga at about ~1 chapter per episode (as well as integrating a flashback chapter in early on) so that pace would mean that it ends right in the middle of a very important story arc, which...can't happen, of course. So either that'll be rushed through or omitted with filler taking its place. Both would be really painful, but a second season is always an option and the filler would really compliment that in the long run.