Your comments

Please add any features (like custom style sheets for preview) you would like to see.

I'd like to know how you use Markdown in your workflow.
I tried to implement this but there were some problems:

The gestures interfered with scrolling and zooming: if you just slightly changed the distance of your fingers, the scroll view would interpret the gesture as a pinch-to-zoom-gesture. You also had to place the fingers for swipe gestures on the screen at exactly the same time or it would be interpreted as scrolling.

So, I had to remove this feature again because it really wasn't usable. I also found those gestures to be quite slow compared to a tap on a button.
I've added some more keys to the row above the standard keyboard in Textastic 2.1. Notably, the "=" character was missing for HTML editing. You can now scroll this row horizontally.

I'm considering to make this configurable in a future update.
This should be fixed in Textastic 2.1