How you think a Crossover should be done

One thing I really enjoy is crossovers. I want this thread to be dedicated to how you think a crossover should be done, and give some examples of Crossovers that you think show parts that work, and parts that do not work, Official, or fan work.

One such fic that I would like to use as an example is One Piece: A Smash Adventure which I feels encompasses several positive and negative aspects of crossovers.

Positive:

• Making sure the characters are in character: This goes without saying, but it's even more important in crossovers. Alot of the fun seeing characters we know and love put in a world you are also familiar with. Having characters act out of character besmirches that a bit. Smash Adventure has all of the Straw Hats in character and the fic so so mcuh better off for it.

• Have characters interact with each other's universe: The best part of crossovers, in my opinion, is seeing the interactions. It's just so fun seeing worlds from respective series through the viewpoints of a different series cast, and the interactions that come between characters.

Negative:

• Having characters from one series overshadow another: This was mentioned by Linkara in his History of Power Rangers Lost Galaxy video. When you have one cast overshadow another, it makes you start to question the competence of the ones being put aside. Smash Adventure has a nasty example in the Mario arc, where Mario gets given a downright insulting Deus Exit Machina, and to a lesser degree in the Zelda and Star Fox arcs. Another problem is that the Straw Hats don't nearly get enough Team-Up fights. Which brings me to:

• Power Creep Power Seep: This is outlined on the trope page, but it can be annoying. This is where I find a team up fight works. It's more acceptable for character from both series fighting togther to do something to bridge the power gap that can come up. Smash Adventure has a few bad examples. Ocelot being able to fight evenly with Robin....Kinda understandable, given the circumstances. Zoro defeating Gannondorf.....Zoro learned a technique but it's kinda pushing it. USOPP DEFEATING GODDAMN KEFKA should NOT be possible, Nerf or no Nerf

So yeah. Give your own opinions and stories and discuss what makes a good crossover!