The smaller the better when it comes to these things. I value symmetry in a post header/logo a great deal. Something small and iconic that gets the job done without being too garish. Here are some I've used in this game and in others:

[hider][center][img]http://s8.postimg.org/pwi2gjdsx/superman_logo_012.png[/img]
[img]http://s28.postimg.org/raf0bi5p5/fantastic_four_by_annikaclarisse_d4uaiox.png[/img]
[img]http://s2.postimg.org/ci5ovlep1/Krybes_4_1.png[/img]
[img]http://s27.postimg.org/5uuwygu27/latest_cb_20130813021947.png[/img]
[img]http://s30.postimg.org/wekde1cel/2000px_Captain_America_Shield_svg.png[/img]
[img]http://s2.postimg.org/dnqcwd7k9/green_lantern_the_animated_series_51671685105ef.png[/img]
[img]http://i61.tinypic.com/qnlmb6.jpg[/img][/center][/hider]
I'd suggest maybe trying to track one down of Doom's mask but some variation of the second option could work. How about this?

[center][img]http://vignette2.wikia.nocookie.net/logocomics/images/5/51/Doctor_Doom_%281970%29_logo.png/revision/latest?cb=20150625210459[/img][/center]