Among the creature types that's beloved but that has yet to get a legend is Spiders. There have been arachnids of some sort in Magic from Giant Spider on out, and that card in particular survived in an unbroken chain of core sets for a very long time, taking a break for a first time only during Magic 2013, and its reappearance the following year shows that it's definitely not gone for good, and that there will always be Spiders in this game.
That's not to say there haven't been opportunities to put a legendary Spider in the hands of players. Innistrad block came and went without including a Spider legend among its many creepy-crawlies - and not even Theros block, with its love of huge monsters and minor propensity for legends, had space for one. Mark Rosewater recently fielded a question about the lack of a legendary Spider in Magic and seemed to give a rather affirmative response.
When rafe62 asked, "Is there any chance you guys could print a legendary spider in a commander set? I know a lot of people who would enjoy this," Rosewater said:
A lot of people who read this blog want one.
Spiders were mostly about defenses and having reach in the early days of Magic, but though that's still a core part of their identity as a creature type they can also do much more now, which would likely make for a more compelling commander. In the comments, kirotherogue said, "Maybe in Black White and Green, the three main colors with high toughnesses and the primary amount of spiders?"