Mute and solo buttons for individual samples in instruments:
I really can't stress enough how tedious it is to have to manually turn the volume down on every individual sample if I'm layering and want to experiment with removing different samples. It's one of the few reasons I still feel the need to use Ableton at all. On paper, it doesn't sound like a huge deal, but it honestly does make a big difference. I know that there's the argument to just put different samples in separate instruments, but when you do that, it creates new problems, like making it more difficult to work with modulation sets, and having to render the sample multiple times if you want it to be played on more than just one key.
Rendering layers to a single sample in the instrument editor:
This is a much less important one, but the ability to render layers in instrument, so that the render ends when playback is finished on each sample, would be really nice. Having to trim off the silent part of a sample rendered from a selection in the pattern editor is a bit of a chore. Definitely not a huge deal, though.
Rendering selections to pre-existing instruments:
Again, not too much of an issue, but it would definitely be nice if, when rendering a selection, there was an option to just add it to a pre-existing instrument. In the current scenario, you have to select the new instrument the render creates, go to the sample page, select the whole waveform, copy it, go to the instrument you want it in, add a new sample, paste in the waveform, then delete the instrument created when it was rendered. It'd save a few clicks and key presses if we could just decide where it goes when it's rendered.