Gosh, I tabbed this thread days ago but didn't ever get around to replying...
I think it depends a lot on what kind of series you're making. Is it action-packed? A fighting game or 2D scroller of it would be cool, or maybe keychains of the characters' weapons, etc.
Romance? Visual novels to sate fans' ships!
And so on.
...but I would also like to bring up Bill Watterson's Calvin and Hobbes, whom most of us know and grew up with. There's a really great documentary on him and his views on Netflix (Dear Mr Watterson) , but what it summed up to was that Watterson consciously, continuosly, chose to not make Calvin and Hobbes merchandise, because he felt it would pervert the idea of his original intentions, characters, the story.
So... you know. It's risky, less income, to not merchandise... but that is one way of doing this, one way of looking at it, as well.
Just throwing it out there as food for thought! 