I'm pretty sure there are other requests for this, but being able to save window arrangements/layouts into custom workspaces is a pretty essential feature. I use Photoshop and Illustrator, but will sometimes use Affinity Designer and Photo for fun. My Illustrator is CS6, so I'll often use Designer to at least convert my AI vector to a more modern/stable copy or just design it in Designer if I don't need to use the Illustrator features I'm use to using.
Having to rearrange my windows every time or make due with the default layout everytime I switch to an Affinity program is extremely cumbersome.
On the whole, I love the programs Serif Labs have put out and think they're extremely functional and promising. It's fun and exciting to see how they're progressing and I hope they continue developing like they've been doing