The Byrd line covers the basics quite well at the moment. Quite a few of the major Spyderco knives have a Byrd counterpart. Delica-Meadowlark, Endura-Carara, etc. While not identical, the spirit of each knife and it's corresponding Byrd remain very similar. I can only make assumptions of sales volumes of each model, but an expansion of the Byrd line should include a Byrd format of a well selling Spyderco.
My own preferences aside, because what I want would be discontinued the same year, it would be interesting to see new models that may be too difficult to risk through Spyderco itself. Something that Sal or Eric wanted to try, but couldn't find the room for in production due to competing projects, or couldn't justify the high cost of tooling or specific processes. Use the Byrd line as the proving ground for something that might be risky, but has potential. A poor selling Byrd knife would cost significantly less than a poor selling Spyderco.
The Byrd line is the budget friendly version of Spyderco, it could be beneficial to try it the other way around. Put the Byrds to work in a matter of speaking. I prefer the ergonomics of the Byrd Hawkbill, but I'd like to see it made in Seki with VG-10, and have a bug on the blade, but as stated above, my own preferences are generally in the minority. Take my opinion with a whole shaker of salt.
"We make a living by what we get, we make a life by what we give."
-Sir Winston Churchill-