Do UNIT and NATO Spell Disaster? - by Margaret Price https://www.fanfiction.net/s/2466162/1/07-Do-UNIT-NATO-Spell-Disaster Margaret’s first Eroica story – a crossover, or should we say, fusion, with Doctor Who. It begins with an Eroica heist … and then, in Chapter 2, the TARDIS materializes, with the Doctor (Four – my favourite!) and an original character, an alien prince called Jason Krystovan. It’s a well-wrought story, with Dorian and Klaus both in character as we know them, within the Doctor Who universe.
The Eternal Major - by Anne-Li https://archiveofourown.org/works/126432 When you stop ageing, and serious injuries heal almost instantly, eventually someone is bound to notice, no matter how hard you try to keep it hidden. What can you do then? Klaus and Dorian are recognizably the Klaus and Dorian we know … but they’ve been given the blessing/curse of eternal life. The source of that is drawn from Smallville, so I guess that makes it a fusion?
The Bludger and the Snitch - by Laura Kaye https://archiveofourown.org/works/51126/chapters/67457 Dorian, Klaus and Bonham are all students at Hogwarts. The universe is Harry Potter.
The Hatchling Job by Sholio https://archiveofourown.org/works/17160989 Dragonriders of Pern, where K'laus ia a bronze dragon rider and Dorian is the son of a deposed Lord Holder who always wanted to impress a golden dragon. It's very sweet. And K'laus' dragon, Ayeth, makes me smile.
Kadorienne's The Picture of Dorian Red http://archiveofourown.org/works/99240 The Picture of Dorian Gray. In which Dorian ("our" Dorian) has his portrait made and lets that picture carry all the strain of his excesses and pursuits. Also with Amanda from Highlander. Dorian steals The Man In Purple, so Klaus has Amanda steal Dorian's portrait ...
Klaus’s Christmas Carol, by dkwilliams https://archiveofourown.org/works/2805275/chapters/6301649#workskin counts as a fusion with Charles Dickens’ A Christmas Carol. Eroica characters take the roles of the Christmas Ghosts from Dickens’ story.