lotus staves are available with most attributes, so you actually can get them easily.
There are other skins that you just can't get unless you unlock the pvp skin and transfer it to a bonus weapon.
a nice skin that immediately comes to mind for me is canthan short sword, which is only available as a green (law and order) that isn't even a sword.
I’d love to see some new armor sets like the Warmaster Grast version of Templar armor. Could add a lot of interest.
Adding on to what Danthor said. I would love to see Eternal Warrior Platemail to complete the Ghostly Hero look without tonic.
Edit: Adding a link to the armor for visibility.
Speaking of the eternal it would be awesome if they added the option to craft an Eternal Blade at the Eternal Weaponsmith in FoW.
Labyrinthine Spire comes to mind, make it drop in NF desolation area
also adding to the wish list:
please make Tormented and Eternal weapons dyable 
(like chaos axe)
skullcrest focus is hella cool
also there is some skins on npcs that can not drop - for example that one handed hammer thing on some weaponsmiths. also bmp or reward skins like deldrimor, imperial, envoy, ... or collector skins like norn/asuran axe/shield/etc, gemstone staff in moddable or os rng version would be dope. but I wouldn't want them to add too many else other skins would become incredibly much more unlikely to drop... I also like the armor idea!
I made an inevitably incomplete list of skins that could be added with some criteria: https://wiki.guildwars.com/wiki/User:Loggy/LimitedSkins
