Challenging for whom? The encounter would need to be completely redesigned from the ground up, complete with fantastically overpowered buffs to keep the solo player alive and able to contribute to the fight, and it's either going to be a complete cakewalk for everybody or too hard for a percentage of the players who asked for it (for example: Gothmog's Rage, which did have those buffs and still proved much too difficult for a pretty large percentage of the audience, while still managing to be too easy for the rest). The team made a really interesting and challenging battle against Shelob (and friends!), and stripping out all the challenge of that fight feels counterproductive to me: she's Shelob, very high in the pantheon of fantasy's greatest monsters, and she should be TOUGH. I do think there's an audience out there who would absolutely eat up a story-focused raiding video series, if such a thing existed, so you can still see how the story unfolds, but watering down the encounter feels like it de-fangs the experience.
The thing is: I love Shelob. She's a major part of my very favorite section of The Lord of the Rings, and I think Shelob slinking away to an uncertain fate following Sam's heroics is a perfect conclusion to her story. Replacing Samwise the Stout-hearted with the player in a solo instance (even with your favorite Elf buddy nearby to provide sufficient cover for the question of 'How can you stand toe to horrible foot with Shelob?' doesn't make me happy. He faced her alone, and that was significant; the player, no matter how amazing (and you're all pretty amazing!) shouldn't be able to do the same. As a designer, I of course know all the counterarguments ('She was wounded!' 'You've done this before!' 'Just have her run away!' etc. etc.), but I don't find them compelling in this case. The team enjoys making solo versions of smaller group-size instances, but "make a solo raid" feels like a bridge too far. Raiding might not be for you, but then again maybe Shelob is the only villain of sufficient notoriety to get you to try it - and who knows, maybe you'd like it.
MoL