Whatever This Is: Experimental Arrangement

compile a large library with orchestral building blocks 

I must say, I'm not a particularly big fan of this "building blocks" theory. Doesn't work for me. All the libraries I have are just data tombs. Before I spend hours trying to find something suitable from a library, I prefer to draw it myself. Currently I'm working on drawing a tune. I'm pretty sure that no library in the world could help me with that.

I once thought it would be a good idea to build up a sketches library. To use Synfire as a kind of accompaniment band for songwriting. So I imported hundreds of accompaniment styles from my old Onyx Arranger software. That took me weeks. How often did I use it then? Never. It's a data tomb. It just makes Synfire take three times longer to boot than normal.

I imported hundreds of accompaniment styles from my old Onyx Arranger software. That took me weeks. How often did I use it then? Never.

I would be curious whether you feel the reason you are not using these ready-made phrases is more of a practical nature (e.g., you already have some idea, and it takes too long to find what you are looking for) or more of an aesthetic reason (e.g., you aim for some motivic consistency, and there is hardly/nothing really suitable in your collection that fits what you already have)?

If it is the first problem, perhaps there are ways to solve it with UI design developments at some stage? E.g., there could be some more advanced phrase search methods. Potential idea: tap a rhythm and list phrases that are similar (or sort by similarity)?  

I mainly have the 2nd problem, and I think this problem is difficult to be solved with a collection of ready-made phrases. Instead, I rather use some of the various phrase transformations  that Synfire already offers. Thanks to Andre and the team for those!

For me personally, it would be cool if I could expand these with custom transformations defined in some API, but I do not expect that to happen.

I would be curious whether you feel the reason you are not using these ready-made phrases is more of a practical nature (e.g., you already have some idea, and it takes too long to find what you are looking for) or more of an aesthetic reason 

Both. On the one hand, it is actually much more complicated and less flexible to use these styles in Synfire than in the original software. Of course, this is not really Synfire's fault, it is simply a different concept. In Onyx Arranger, these styles were immediately accessible, allowing you to tweak a number of parameters, insert fills and breaks, etc. In Synfire, it takes several steps to use them: You have to open it as a sketch first, if you then want to play it with a palette you have to press "Play this Sketch live", and then if I want it to have in the arrangement you have to convert it. Then it opens as a new arrangement. If I then want to have this in my actual working arrangement, I have to move it over somehow (for which there is unfortunately no longer a function in Synfire 2). If I then want to try out the next style, I have to do everything again. 

But to be honest, that's not the real reason. The real reason is that you can't come up with these styles these days. They are all really well made, highly musical, you can tell that a lot of work went into them. But they no longer have any place in my tracks. Using predefined styles was fun and practical in its days. But if you want your own tracks to sound fresh and new, and that's my aim, then that workflow is no longer an option. 

Years ago, I did something with Yamaha styles combined with slip composing technique that I applied in Synfire.
Could make some songs with it that didn't take too long to make.
Could make musical-sounding transitions between instrument blocks with it and that's what matters. 
Were more random compositions , determined by the yamaha styles and you want to make more of your own songs anyway, but that requires musical knowledge of song styles
I am now also working on making an (existing) melody myself, but that is not as easy as it seems.
Gives you the satisfaction of making your own composed melody, though.
Unfortunately, there is no midi of it, so I have to make do with an indistinct screen impression, but that is instructive 
More convenient indeed to modify existing piano phrasing a bit.
to arrive at your own sound.
But the aforementioned self-created piano melody will not be easy to find in my piano library, or probably never will be
You can get pretty far via phrase harvesting
Limiting yourself to a certain genre also makes it easier to compose. 

Could you share your libraries (data tombs)? :)

Maybe they can prove helpful to other users...

I am also curious if I would use the sketches @Jurgen has collected from Onyx arranger so loaded into Synfire.
What that means if I start composing bunch with it like I did with the yamaha styles, because there I just imported the Yamaha styles. 
Am concerned with the accessibility in Synfire of the sections of the styles. 
Maybe I can also load a Yamaha style or whatever style ( band in box ) as a sketch?

I'm cautious about that. I think that's still copyright-relevant material. The company also seems to be still active. Another user recently told me that they still respond to support requests. So better not