Some ideas gathered from comments:

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4x4 and 8x8 size variants of those round tube caps, 3-way and 4-way intersection variants in 4x4 tube size

Good idea, might make eventually

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sort of an egg shaped elongated variants of tube caps

probably wont make, prop resizer can be used to achieve this.

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those conical shaped tube pieces (i.e. 4x4x2to2x2) in 8x8 diameter

Maybe.

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and circular plates with a hole in the middle so they can be used to connect two differently sized tubes to look like one thick walled one.

Technically exists now that I have added the arches. Have to use 4 of them tho so might be good to add a complete circle.

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a greater variety of triangles/ discs would be a great addition as well

maybe, will need to investigate which triangles would be needed.

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cones would be nice

Could be usefull will maybe do them eventually.

window pices based on the larger tubes would be nice
an 8x8x4 curved window would be useful in builds

some more wood props would be nice

I have an idea, what if you could make a version of the tube4x4x2to2x2 with windows of different sizes?

A way to know which props are from this addon, maybe you could make a new subcategory in the plastic props category? I dont wanna try using it on a server but can't because they dont have the addon and I accidentally used one of your props.

can you please make flat circles with a hole at their centre pls

1x discs and windows to go with the 1x squarehole
organized spawnlist

expanded triangle parts