For practical purposes I think about something in line of Cessna 172. As it was the most produced aircraft, then it must have hit some sweet spot or be near it.
- atmosphere with barely noticeably higher oxygen content, and much higher nitrogen
- tech level similar to contemporary
- rugged, simple, inexpensive aircraft that can serve well on sparsely populated planet
OK, so how to adjust it for such planet?
Assuming that I get physics correctly, in order to keep the same performance, it would need roughly the same engine, frame, but wings would need just 1/3 of surface area from Earth. So my first guess would be a Cessna clone, but with trimmed wings... ok with much smaller wings.
Honestly, does it make sense? Or maybe under such lift friendly condition this wing size reduction offers minimal gain and when having such leeway there would some clearly more tempting things to improve instead for an utilitarian bush aircraft? (Dunno minimum take off distance, load, whatever?)
Question: Is such wing surface reduction correct way of making rule of thumb aircraft adjustment for thicker atmosphere? Assuming "Yes", does it make much sense for its intended function?
EDIT: Extra food for thought: Does such aircraft actually need normal wings at all? Seriously. Under normal conditions small part of the lift comes from aircraft body. When the need for lifting surface is reduced, then the share of the lift provided anyway by the body would increase, but I'm not sure whether that would be significant enough to matter much in design.