I've kept my dipslay of ML to two shelves, both about 18" wide and 18" deep. The main shelf is all Avengers from Iron Man/Giant Man/Hulk/Thor/Wasp to the West Coast branch to some of the New Avengers.
For my display, I'm only using one of each character. So if you could bring all of the Avengers together, this is how it might look, but you wouldn't have a phasing vision and a regular one or two versions of Wonderman, 6 versions of Iron Man, etc. You take your favorite of each character and put them in the scene. My "extras" of each character (Extra Thors, Iron Man, etc.) go on the second, less prominent shelf.
I'm a little random in my selection - I included a custom Diamondback figure from Captain America, but didn't pick up the Luke Cage figure that came out. I have her on the first shelf and Arachnia on the first shelf, but Beta Ray Bill is on shelf #2. I also have both Yellowjacket and Giant Man on my top shelf, and I had to include FA Iron Man and the 80's Gold and Red Iron Man on the top shelf as well, so its not perfect.
The nice thing about this is that its almost complete. I'm having a custom Tigra and Mockingbird created by a guy at Fwoosh who made an awesome Kate Bishop for me. I'll probably get around to adding US Agent, maybe Sersei, Crystal, Starfox, and Quasar, and if a better Luke Cage comes out, I would add him. Possibly Beast as well. Not adding all of the New Avengers nonsense that's going on now - Iron Fist, Dr. Strange, Iron Spidey, Wolverine? Those guys do not belong in my avengers, though I did squeeze Wolvie into the top shelf for now.
If you wanted an assembly of the best of Marvel, I would do something similar to the one version of each character and just put in the major characters/your favorites. For me, that would look something like:
8-10 Avengers (Original team plus Witch, Quick, Hawkeye, Cap, Herc, Black Panther, Black Knight, War Machine, Ms. Marvel)
10-12 X-men (Original 5 plus Wolvie, Colossus, Storm, Nightcrawler, Rogue, Gambit, Bishop, Cable, Prof X, Kitty?)
Fantastic Four
Spiderman
Daredevil
Dr. Strange
Moon Knight
Punisher
Ghost Rider
Captain Marvel
Any big names I forgot? If you've collected all of the bad guys as well, would be pretty cool to set up a display with the good guys charging right and the bad guys charging left - I mainly just collect the heroes.