Hey Gk,
I too am in the process of constructing my next demo reel. What Danny said is right on the money, I'll just add my experiences as it may help you and others regarding demo reels.

Length:
As Danny said, keep it short. My first demo reel(which was awful) was 4min long. When I see now, it was way too long, and it seemed longer because it was soo bad

As I began planning my next demo reel, I was thinking of making it 2.5 mins long, but once I started on projects, even that seems like a lot of time. My demo reel will now be between 50-80 seconds. Here's an old saying that definetely applies here... Quality over Quantity. If you only have 30secs of good animation, then only show 30 secs, don't add an extra 2mins of tutorials you might have done to make it longer. That won't help, instead it will hinder you.

As to clips, and how long each clip:
Well, I'm planning on doing 3 clips. One 20sec, one 10 sec, one 25 sec. I have two others that are about 15sec each, but I'll not do until the first three are done in great quality animation.
Even now, I feel some of these clips might be too long...

Take a look at these great animators and see how short their clips are, but with great quality animation in that short span.

https://www.cameronmiyasaki.com/Anima...n_frameset.htm

https://carlos.mclinn.net/animation.html

https://www.keithlango.com/

Keep it short, keep it short, keep it short. I still need to remind myself of this.

Re-creation:
I'm an avid Star Wars fan. I've asked about my demo reel having a sw theme kinda thing, but most frown on that. Mainly because it's been overdone, might show lack of creativity, it'll be compared to the films, so then must match the quality which won't be likely since it was done by a group of professionals
With that said, if you're still set on having a Matrix theme to your animations, try hard to make it your own, and not a part of the movie. Like the animatrix flims, they have the matrix storyline, but not exact from the film. Put your story to it.
I might throw in a light saber in my demo reel

Music:
Like Danny mentioned, a lot of people turn off the audio, especially when looking at animation demo reels. If you think about the old black and white films like Charlie Chaplin, that's what they want to see. Your animations being able to express feelings/moods/weight/acting without the aide of sound. If you can do that, you're in. Hehehe, again with that said, good choice in music may help your creativity... Just make sure it's in good taste, doesn't take away from the animation, and like Danny said, make the music Secondary.

Final note:
To make sure you get the best quality out of your animations, it's a good to keep hanging out here at simplymaya.com. As I'm sure you know, lots of great people that can help you with any questions you may have, and also critique your animations for positive feedback to insure that quality.

For more info on demo reels/3d industry, check out these tips from
Jeremy Cantor/SONY PICTURES IMAGEWORKS animation supervisor:
https://www.zayatz.com/pages/tips.htm

Hope this helps,
God Bless,
George