Here's a possible compromise: single fandoms are still nominated (maybe without the "seconding" requirement), and then participants are allowed to put a minimum of two fandoms in each request and offer - but can still put more.

As someone who's mostly interested in specific fandom pairs being combined, this sounds a good, workable compromise to me. It allows those who enjoy the bucket offer system to combine three or more fandoms and keep the surprise and the mystery and the challenge, while also allowing those of us who'd love to see fandoms A/B or A/C but not B/C to avoid getting a gift that would disappoint us (or having to write something we absolutely don't want to write).