here's what i came up with after some sperging, see picrel
- the leftmost one is a normal diverging diamond, with its characteristic traffic going the opposite way thing. it's not that good on its own, as there are too many crossings, and the usual benefits of a DDI don't really apply in factorio since trains don't ever crash, the middle buffer section would only ever really be able to fit a single train, and it would become a massive pain to have to wait for trains to cross when they're simply trying to go straight (it also should be taller to fit at least a 1-4 train in the middle section, this one can only fit a 1-2 but it's an easy enough fix). suffice to say that it could be better-
- the middle one is basically >>543547649. a normal diverging diamond except that the opposite lanes don't cross and instead just go over/under each other, which makes it so trains going straight don't have to stop for one another. this one still has the one crossing when trains from two adjacent directions are trying to go left (e.g. from north and east), and so the fix for that is
- the rightmost one, a full double crossover merging interchange (>>543548693). the point is to have no useless conflict points at all: trains going left or straight don't ever have to stop for each other and the only time they ever would stop is when trying to go merge to go into the same direction, which is the case for any interchange, so in theory it's as good as it can get when it comes to crossings.
some quick blueprints if anyone is interested: https://ghostbin.lain.la/paste/zzmce