Hello!
Landscape mode: Yes there is a plan for that, currently i am very busy improving the invoice designer though so this feature might take a while to be implemented =).
Regarding text editing as html: Currently i am trying to avoid that and the reason is that converting html to pdf is not very accurate, there are a lot of things that work in an html page but doesn’t work once you convert it to pdf, or work different (for example absolute positioning of elements or floating tags).
Currently the plans are to keep improving the designer so you can do everything (or reasonably everything) without the need of updating the html directly and the plugin will internally make it work.
Regards!