Hey -
TR;DR The RPG community has a wide variety of degrees of Openess that serve different purposes. I will let you judge what is best for your use case. I myself like CC Pro-Commercial Not-Require Share Alike Attribution. You can totally have an open game that you charge for.(More on that below)
So there are degrees of open that the RPG community has used. (Really high level) The Basic Roleplaying Game System has a license that prevents the users from using Insanity mechanics (and insanity by any other name) in their games. Why? Because the creaters what to protect their work Call of Cthulhu and so they released the core of the system on the caveat that you would avoid that specific mechanic to protect them.
Others like Fate have released most everything, in part because the system was made to work for everything and the protects made from the system weren’t tied to one genre.
Some people like to require Share Alike. Others don’t require Share Alike. Every open game has found a way to protect something (as they should). So D&D had the OGL but didn’t apply it to their books they made a System Reference Document that specifically excluded (among other things) Beholders etc. That way they (old Wizards of the Coast) could share what they wanted to share, but keep enough to protect themselves.
Here’s the caveat, the more your license “looks over the shoulder” of the people using it, the more people will be nervous about it. FATE (I believe) somewhere (I need to find this . . . ) talked about why one of their licenses didn’t require “Share Alike”. They wanted their system to be used with characters etc. that ppl wanted to protect. If they required Share Alike people may be nervous as to whether there was a reasonable way for them to make use of the system without losing their characters.
I like (personally) CC Pro_Commercial, Not Share Alike Attribution. Because I feel that, as long as I am attributed, I have already have protection for my original. People will think “Oh cool this cool thing was forked from this . . .” I don’t want to forbid commercial, and I don’t want people to worry that if they use mine they will be forced to share IP that they, for their use case, can’t share yet. (Or won’t ever share yet)
Some people, really wanting to remove any barrier from their ideas getting out there, have even Public Domained stuff. I (myself) have never done that:
If I have a Open House at Christmas. I am inviting (my friends and family) to my open house. But not necessarily my private bathroom / bedroom. So an “Open Game” (which will be for more than your friends and family) does not mean you have invited ppl to use everything. And in my case, the invitation to my “house” includes the attribution requrest. Just as your invitation to your (real) house/apartment may include a “take of your shoes on the carpet request” Having (reasonable) strings does not mean you aren’t being open.
“Under a Hostile Sun” in its final form will have a lot of Campaign Setting stuffs I am not going to include in the SRD.
So if (I don’t think I will but if) I wanted to Charge for Under a Hostile Sun, I would make the SRD free (because its licensed) but the storylines I wrote aren’t, and thus by extenstion the specific way I use these rules for the specific storylines I wrote are not licesned either.