I was revisiting some discussions you and I have been having - One of the pitfalls we ran into was changing subjects in the middle of a disagreement.
So, for example, one rule could be "We can't change subjects. If we disagree on A we keep talking about A without moving to B or C".
To help us get focused on what we disagree on we could 1) keep it specific (for instance, rather than saying "Trump bad/Obama good", we could discuss policy details) 2) keep a small, one-liner in bold of what the disagreement is at the top/bottom of each post 3) refer back to the one-liner when we feel we are straying too far from the path.
This is just an example.
Another example could be to refer to sources we both agree are fairly reliable ones. This statement, as it is now, ("refer to sources we both agree are fairly reliable ones") is too generic though, because, even if we agreed it's a sensible one, we'd also have to agree on a set of common sources. In other words no point in me thinking source X is reliable but you disagree and you thinking source Y is reliable but I disagree.
A third example might have something to do with avoiding fallacies. We could even devise a score system whereby we lose points by using fallacious arguments. Once again, we would have to agree on fallacies too, otherwise the danger would be that either of us points out the other just used a non sequitur argument and the other disagrees.
I could go on but hopefully it makes sense.