in Preferences --> Wall pattern in plan it would be nice to see cross hatching as a fifth choice. Especially for diagonal walls this would be useful for better visibility, without using too much ink/toner when printing.

No problem, it will be available in next Beta version.

In earlier versions it was possible during Import background image to click in the top right corner of the screen to enlarge the import window to (almost) fullscreen. In v2.6 and v3.0 this isn't possible anymore. Can we have it back?

I didn't change anything here. Java guys seem to have made the choice to remove the enlargement button on dialogs, but you can still enlarge the dialog with its borders.