Beyond individual issues, are there other X-Men related collectibles that complement a comic book collection?

Absolutely! X-Men action figures (especially vintage ones), statues, trading cards, original art, and even movie memorabilia can complement a comic book collection. First edition novels based on X-Men storylines, and promotional items from the comics' publication era are also popular. Building a collection around a theme (e.g., Wolverine, the Dark Phoenix Saga) can create a cohesive and impressive display.