+1 BIG TIME for data merge. Frankly, without it, there is really no reason to move to an affinity publisher platform from my current workflow.
To date, Designer and Photo have served my needs very well. I started with Designer and after a few weeks use there, Photo was happily purchased. If Publisher debuts without data merge, I will just have to wait patiently until the feature is added; Publisher would will not be an automatic purchase.
I truly believe that you will be shooting yourself in your foot if you launch a software application targeting professionals and advanced hobbyists/amateurs without a top-notch, innovative, and user-friendly data merge capability. In the circles I create, this feature alone could set you apart and drive demand from hundreds of users to your software.
Best of luck with your project.