My opinion is of course everything should be open, but let me take a look at it from a more pragmatic point of view:
If your basic premise is to make money by licensing the work, then obviously you must keep the rights. In this case, please, don’t do what Arduventure (Arduboy game) did – the plot and the characters, which they reserve all rights for, are mixed with the code, which they licensed as MIT. That’s very, very bad, it’s not at all clear where the line between code and the rest is drawn. Rather make a separate folder for code and for the assets and say this one is MIT and this one is all rights reserved.
If you want the characters made open, but would like to keep the rights “just in case it becomes real succesfull”, then I would say this: The outcome is almost certainly to just be a restrictive and more discouraging content for the community, because the work most likely won’t ever become successful enough to have a significant commercial potential – that is extremely rare in case of random works of individuals. For a work with commercial potential you usually need to invest money, explore the market, make some plans etc. So – if it’s just “I want to keep this lottery ticket”, I would really advise to throw the ticket away and rather open the content for the community, that will almost certainly bring you at least an appretiation of the community in return – which can actually lead to you making money as well if you really want to and establish yourself this way – it’s slower and more work, but more real than betting on a very slim chance .
Open characters will also encourage the merchandise creators and sellers, because they won’t have to pay for the IP – that means no money for the creator (you), but don’t forget that it’s not just the IP that’s making them money, it’s mostly their added value – the manufacturing, advertisement, running the shop etc. So it’s not like they’re robbing you, you would have to make this extra investment as well if you wanted to make profit on the characters. So no need to feel bad about this, you’re at least getting free advertisement of your work from them.