Nothing wrong with the size you have. It's safe and perhaps conservative for a 200-series, if that's what you're getting at?
Once upon a time, a factory approved tire size was 285/70R17, or 32.7" diameter tire (your 275/65R18 measures 32", stock 31.4"). So you can see there's ample opportunity to go up.
As you've suggested you're going down the path of rack and bumper, it sounds like you may be wanting something on the more aggressive end of the spectrum? A 285/60R18 will be a better and more proportionately sized tire IMO for the mods you're thinking. The larger tire may have some nice added benefits of additional clearance, more sidewall for a bit more ride quality and off-road compliance. Perhaps with a small hit to MPG.
There's opportunity to go further still, to almost a 34" tire size with relatively minimal mods if that interests you.