Most of the ones I've watched are DC based with a heavy bias for Batman. I have a playlist on Youtube of many of my favorites. The first video on that Playlist is one I wrote myself, I had the idea in my head for awhile, but I have no real video editing skills, but because I made a thread on the subject on IMDB SolyentBrak1 wound up editing the idea together for me and he did a great job. I have other Brak fan trailers on the playlist as well.
Most in that playlist are Fan Edited trailers, but there are a few full on Fan Films that are really just trailers like Grayson.
I first discovered them in the era of trailers being made to speculate what the third Nolan movie would be, The Riddler was popular.
I've seen many more then what I've put on the Playlist. I want to mention a few things some do that for me personally at least take me out of it, and so I'd recommend avoiding.
Using any audio from Animated projects, whether voice over, dialogue or even music. Even if I haven't seen the animated project being used it always sounds like a cartoon to me. Plus I want the voices to match the physical actors. Whether it's using voices from the Dark Knight Returns cartoon, or Flashpoint, or even Amanda Waller from Assault on Arkham or the DCAU, that actress could be a good live action Waller, but from those projects her voice always sounds like animated voice acting. So for DKR or Flashpoint it may be an easy way to get direct quotes of the Comic in your trailer, but it's just not worth the awkwardness. The worst offenders are those Batman/Dracula trailers that use the narrator from the commercials for The Batman vs Dracula. The best Batman/Dracula trailer is the one in my playlist with Burton's Batman and Hammer's Dracula.
Every rule has one or two exceptions they say. In this case a lot of people would like to see Jensen Ackles as a Live Action Jaosn Todd/Red Hood, and in fan trailers based on that premise using Jensen's voice work from the Under The Red Hood animated film tends to work. The voice does match the actor, and I think also Jensen simply wasn't doing it like usual voice acting, which would normally tick me off in an animated film but his performance was good. However even in some of those if any other actor's voices from that same movie is used it ruins it, I like Bruce Greenwood's Batman voice, but I don't wanna hear that when I'm looking at Ben Affleck or Christian Bale.
Also using visuals from Video Games like DC Online or Injustice, or the Arkham games. Unless it's something that absolutely would be fully CG in a LA film anyway like a Darkside or Doomsday, or maybe Brainiac, that just never looks right. And even then it needs to be a really high quality graphics video game, PS3 level at least. But I've seen trailers do it for Batman and The Flash. And using the Black Mask or Deathstroke from the Arkahm Origns trailer is really popular. To me it always stands out awkwardly.
It amazes me how often someone makes a bad decision in the music/score department. There are now a few Superman/Batman trailer premised on it staring Christopher Reeve and Michael Keaton. Most use neither's Iconic score, one uses the Williams score but for Batman uses the Nolan theme, that simply doesn't match. Keaton without the Elfman score works sometimes, but if that specific nostalgic crossover is the gimmick, I wanna hear both Iconic scores.
Those are my constructive criticisms. Now I'm going to throw out into the ether some ideas I'd like to see that haven't been done yet, not that I can find anyway.
The potential is out there to make a decent Dark Knight Returns trailer using material from the Nolan Batman films and Batman V Superman. They all take some elements. Begins has thanks to the fear toxin the mot surreal moments, TDK has the gun touting Batman copycats. Rises has the cop saying "you're in for a show tonight" line, and Alfred talking about Bane being in his Prime is similar to what Alfred says in DKR about the Mutant Leader. And BvS has Batman fighting Superman with many of the same elements, and a nuked Superman being healed by the Sun.
In costume which Batman you use shouldn't matter, but I'd favor Affleck who looks older for Bruce Wayne, yet Michael Cain for Alfred due to the line I mentioned. And Snyder's take on the killing of the Waynes and falling into the cave fits better, just leave out him flying. Only appearance of Harvey Dent I'd put in the trailer is him saying "There is no Escape from this". Also throw in stuff from the Gotham Tonight special features segments to help get the whole Batman in the news vibe going.
Leto's Joker would fit this more then Ledger's. Use scenes from Suicide Squad trailers, or that Music Video. Use the Gotham skyline from Gotham on Fox since it has those twin towers similar to yet distinct from the World Trade center. Include stock footage of Ronald Reagan as President on TV. And throw in the Bat Demon from the bat-cave in Batman Forever and it's deleted scenes, also use Forever's establishing shots of Arkham. And you could make a pretty convincing DKR live action trailer without any need to cheat by taking audio from the animated movies.
A few cool Teen Titans fan trailers have been made, they can show us where to start.
But my main disappointment is none are able to include either Wonder Girl. You'd have to be creative but you can find a way, there was a time when we had Gal Gadot as WW in fan trailers when we hadn't seen her in costume yet. I want a trailer that looks like either the Wolfman/Perez 80s Teen Titans, or the Geoff Johns 2003-2006 Teen Titans line ups. (I don't want just the animated series line up.) Cyborg, Raven, Beast Boy/Changling and Starfire (and maybe Jericho) are part of both those, they go from being the new team members to the older ones.
Troian Bellisaro has become an ideal pick to me for Donna Troy. I haven't seen much of what she's done outside Pretty Little Liars (just the two episode of NCIS). But I know on that show in Season 4 Episode 1 we see her investigating a burned down building, in a TT fan trailer that could easily make fans think of the orphanage in Who Is Donna Troy?
There was a TV spot for that old Cartoon Network show that had what looked like a convincing real life Titan's Tower. Perhaps you could throw that in.
But you have to use the Arrowverse Deathtroke over Smallville's horrible one, (or maybe the DCEU one that is now coming soon, but on Arrow he interacts with potential Titans characters).
The existing trailers have a tendency to mix and match the line up, I'd prefer to avoid that also. If the Robin is Dick Grayson then Donna should be Wonder Girl, Wally West should be Kid Flash (we have him in Costume now from the Flash season 3 previews) and Roy Harper as Speedy/Arsenal, and only this older lineup should have Terra. If Tim Drake is the Robin then Cassandra Sandmack should be Wonder Girl, Bart Allen should be Impulse/Kidflash, Mia/Thea for Speedy and have Conner Kent/Kon-El/Superboy. Bart and Conner would have to be taken from Smallville.
I used to think the ideal Batman Beyond fan trailer needed Clint Eastwood as Batman, but he's more DKR Batman, which I do prefer to see as distinct.
There is a trailer on my Playlist for a Dark Knight Returns film starting a contemporary Michael Keaton. It's cool but doesn't feel like it references anything specific from the comic. And Keaton simply isn't a harsh enough character in any role for that.
But it got me thinking that a version of Batman Beyond with Michael Keaton could be cool.
I still like Stephen R McQueen for Terry McGinnis (and maybe Chad McQueen for his father), Malese Jow for Dana Tan but perhaps Janel Parrish for a second choice. And either Jessica Parker Kennedy or Zendaya for Max Gibson. And Sasha Petierse for Chelsea Cunningham.
Angie Harmon for Barbara Gordon because she played an authority figure on Law and Order, so maybe she could be the second rare exception after Jensen Akcles. In case you don't know she voiced Barbara in Return of The Joker.
Not entirely sure what to do for The Jokers.
Perhaps Cameo Appearances from modern Kim Bassinger, Michelle Pheifer, and Billy Dee Williams, or the actor who played Knox if he's done anything recently.
I don't have a compelling suggestion for Derek Powers or his son just yet.
I like the idea of taking the original The Dark Knight teaser trailer and replacing it's blue tint with red. The Nolan bat-symbol is similar in shape to the Beyond one.
Some Batman Beyond fan trailers have already started using Beyond Fan Films for Terry in Costume.
Last but not least, Crisis on Infinite Earths or Infinite Crisis or anything vaguely like that. Multiverse ideas.
These became more popular over the last year, Season 2 of The Flash provided the key exposition source material needed. And some made already I quite enjoy. But none have used George Reeves. I want him as the Earth-Two Superman, he was in color in the latter seasons. Earth-Two Superman is the most important, he, Kara and Barry make the key heroic sacrifices. George Reeves was a WWII vet and had the right basic look by the time the show was in color.
And Lynda Carter is naturally the Earth-Two Wonder Woman.
Some more of my thoughts on the DC Cinematic Multiverse I discuses in my last Batman V Superman post.
And with no particular idea of what fan trailer to feature her in, I've recently thought of Ashley Benson for Stephanie Brown.
So those are some ideas I'm throwing out there. Feel free to use them. Let me know about any Fan Trailers you've made or seen you think I might like and if I do I'll add em to the Playlist and promote em.