I've seen suggestions for this ever since Exile released his CKF Remastered mod which notably lacked the right triangle parts that existed in CKF Legacy. These suggestions migrated over to the Arkitect Structure mods after they were released. I'm writing this so I can just refer people here when the suggestion inevitably comes up again.

I know some people want right-triangle parts. Some people might even hate the equilateral triangle parts, even if they know how to use them properly.

Adding right triangle parts would not be a quick or easy task. The diagonal side of the right triangle parts is longer than the other sides so none of the existing models for walls and roofs would fit without remaking them. A lot of models would have to be remade to work with right triangle stuff.

I'd also have to set up the base classes and all their snap points, etc. Snap points, as any Ark modder who has dabbled with structures will attest, are a royal pain in the ass even once you understand how they work. I still find snap bugs even months after the initial release of these mods, and after loads of testing. It isn't so bad when you're just dealing with a handful of simple structures that snap to one or two things. In full blown structure mods, a single base class can have 50-100 snap points and there are a lot of base classes in my mods. One for every shape, basically. It's a brittle system that is tedious to set up and easy to break.

There is an inescapable tedium that comes with adding any new parts: setting up the structure BPs, their items, making the icons, setting up all the details like crafting costs and testing to make sure there are no mistakes. After making well over 400 structures so far, I can say I greatly dislike that grind. I mention this because it is a ton of work on top of the specific work required to make the right triangles and diagonal stuff.

Lastly there is the time spent in Blender, the 3d modeling program. Some structure parts can be easy to make, but a lot of them take significant amounts of time to get right. Especially the tile roofs from Atlas. I've only been learning Blender since May of 2022, and despite learning tons, it's still a slow process for me to get the models looking good in some cases.

There are currently 13 tiers of parts between AS:Core and AS:Atlas (2 thatch, 2 wood, adobe, 2 stone, metal, greenhouse, tek, and 3 glass tiers). Adding even one new part usually means making it at least 12 more times. Even more times if a part has cosmetic variants, and some of them currently have as many as 20, with more planned.

Right triangles wouldn't add just one new part either, they'd add multiple. Off the top of my head it would be at least: foundation, tall foundation, ceiling, wall, half wall, rail, door frame, double door frame, window frame, diagonal roof.

So we're looking at 13 tiers x 10 shapes = 130 models bare minimum, not including variant models and parts I forgot about or could later discover I need. I suppose only 10 of the tiers would need foundations since none of the glass tiers have them, but still. Including all the variants it's easily over 200 models.

That's based on what exists currently. I'm still adding parts, some of which would need diagonal versions.

Hopefully this gives you an idea of why I'm saying no to right triangles.

All of this applies to round parts too, by the way, except round parts would be even harder to make, and have even less bang-for-the-buck than diagonal parts since they can only ever be used for one specific diameter of towers.