A trick I use a lot to make line-art look a little less plain is to add small imperfections and rumples in the lines. Tiny squiggles and ‘scratches’ do well. Or to break up a line by erasing some parts or to even draw in a second line beside it to make some parts look sketchy. I dunno, just something I thought of off the top of my head. ¯\_(ツ)_/¯