The most common upgrade tends to be a better headstrap as the default one is often found to be quite uncomfortable. Meta offer the Elite strap but it's quite expensive and has a long history of fragility. There are numerous well-regarded 3rd party straps available from companies like Kiwi Design and BoboVR.
Next on many peoples list is better battery life. Some addon headstraps include an additional battery or you can simply attach any common USB battery pack with velcro straps or carry it in your pocket.
Depending on the games you expect to play you may find controller covers useful (if you think you may accidentally punch walls or other solid objects) while some people like gun stocks to which controllers can be attached for that authentic rifle feeling.
If you are a spectacle wearer you should consider prescription lens inserts as an alternative.
