> But I think these symbols are extremely out of any style.

My intention wasn't to use these icons in particular. It was just a quick mockup to have a visual example. I was expecting that maybe someone with more skill did instyle icons ;)

> What's the first one? Pickaxe? Tools Workshop would be my guess.

And Tools Workshop it is! You did the trick by just knowing three simple gameplay mechanics: There is a pickaxe, there is a tools workshop, pickaxe is crafted in the workshop. But I'm also fine with something else. A crossed hammer and axe?
The mental trick of linking the picture and the building is necessary nevertheless because the picture are so small that the models are distorted anyway. I can't see the anvil or the tools in the tools workshop picture as well, so gain there. But I'm always mistaking one building for another. Wind Generator and Windmill are looking very similar, too. It's easy to pick the wrong one. Here are two examples of how you could design two different icons:

(again merely suggestions, not finished works)

Also, in addition (was has been mentioned by other people by now) you should get the name and/or description of the building (because no picture will be as specific as that). But experienced players should be able to pick the right building right away without looking twice. And yes, coming up with an icons or anything will be something you have to put some thinking into. But you can't avoid that, regardless of any visual representation. It just shouldn't inferfere with the design of the model itself (because it's meant to be rendered big).

I don't think making up arbitrary categories will help with this. This might do the job for pluto's 40k buildings from his third party pack. We could stuff all buildings from Objects.ocd into a "Original" or "Standard" category and leave it to the developer to sort new buildings from a different pack into their own category (so your buildings menu would then pop up and give you the choice of either "Standard" or "SciFi Hazard MegaPack" buildings). When there's but one category ("Standard"), the menu will directly go to that without impeding you by selecting a meaningless category. And yes, I do think the category would be meaningless. Even selecting appropriate ones for the currently existing buildings is very hard.

"Energy" was the easiest choice at hand because that one's cristal-clear. Or is it? Compensator, Steam Engine, Wind Generator - yup. But what about the Flagpole? Is it "Energy"? Or is it rather "Home"/"Base"/"Economic"? The system fails as soon as a buildings has more than one distinct function.

What about producers? Are they easy to sort? Armory, Chemical Lab, Inventor's Lab, Kitchen, Shipyard, Tools Workshop. Sounds good to me. But then, maybe the Kitchen should be in "Food" because that's probably where the Windmill will be as well. And on top of that, isn't it our task to also predict the categories developers will most probably need? Better split it up into "Weapon crafting", "Tools crafting" and "Chemicals" because it's very likely, some future packs will also have such buildings. But then again, these categories are mostly filled with just one building by default. That's not good at all, nor is it to just stuff every producer which will ever be into one category.

And proceeding onto the more complicated buildings: Which category is the elevator in? "Transportation"? Together with the pump although the subjects of their transport are completely different? And what about the Foundry, Loom and Sawmill? "Refiners"?

No, I don't think categories will make this easier. I do admit, I find them appealing for separating original and custom content but apart from that it'll just be another tedious click to the skilled player and a pain in the head to do right right from the beginning.

> Can you move it?

No but someone else did!