I love the wandering trader and I believe the best items to put in his pool are farmables that can be spread further by the community, items from /warp grass at a discount, and rare items at competitive prices.

Here are some examples I don't think deserve a standalone suggestion:

• turtle eggs
• glow lichen
• glow berries
• dripleaf
• sweet berries (currently in /warp grass, I think it could be better here)
• sea pickle (same as sweet berries)
• wither roses (currently farmable via wither, it would be nice having both sources)
• dripstone
• echo shard
• disc fragment
• elytra
• beacon (currently too expensive to compete with player shops)
• debug stick (yes )
• dragon head
• dragon egg

Some of this items may already be in the pool, but we don't know it fully yet, and still I think this are the items that best fit the new mechanic.

Edit: another item that's missing from the villager's inventory that I think is a no-brainer is Music Disc - Otherside

 

Thanks for your suggestion, the wandering trader was trading demo items during the beta test so the real trades will be different.