Some suggestions for the technically inclined:
1. Scale - this would be so useful for the users who want to make use of designer for technical drawings. I know your artboards can be large (45m by my testing), but when the performance suffers and the grid is limited, scale is a requirement. Look at Graphic app - a good start. And reflecting scale numbering in your smart object indicators would be a huge win.
2. Decimal point accuracy - this is in sore need. When all I can get is 1 number to the right of the decimal point (in many instances), this app is unusable. For years, Illustrator was only accurate to 2 decimals. It sucked. 3 is an absolute minimum, and 4 or 5 would get Designer out of the dark ages.
3. Rulers - a small ask, but judging from the forum, repeated many times. It provides context. Again, tying the scaling to this would be a huge win.
So much of this app is incredible out of the gate - your approach to tackling huge challenges in writing this for a tablet is truly impressive. Keep up the great work at Serif - all of you are setting the bar for a new era in app development. I, for one am a huge fan.