Unfortunately, that's just kinda how it goes for most TV shows with a specific niche. Once the main story is complete, it can seem like the new story lines are stretches. Once and Vamp Diaries have been doing this, but I just try to get over it and readjust my perception to enjoy it for what it is.
Or, you get the complete opposite where the same damn story is incomplete for several seasons...like Pretty Little Liars. I, again, adjusted my perception, got over my gripes, and I now enjoy it for what it is. I think we're on season 5 and there are still unanswered questions from season 1. lol
In good long running series it's good to have a clear meta story in mind before you start. Lot's of writers seem to do it as they go.
Supernatural for example had a good meta leading up to season 5 when the series was supposed to end. After the continued I really have no idea where they've been going.
Once is similar. I really don't they had anything planned for after the curse was broken at the end of S1.