I think Steve Trevor should be in Wonder Woman's solo film, even if it's just a small appearance (depending upon the requirements of the script).
I'm not sure who should play him until I have a better idea about Wonder Woman's personality. I think Steve Trevor needs to have elements that balance against Wonder Woman's characteristics. If Wonder Woman is a one-dimensional ass kicker, as she has been in many of her New52 appearances, then I'd really want a lighter, funnier and generally more upbeat Steve Trevor who can enable Wonder Woman to show a broader and three dimensional character.
If Wonder Woman is an ass kicker who is also peaceful, adventurous, compassionate, inquisitive and humorous, then they could approach Steve Trevor in a different way. He could be very much a military man and Wonder Woman convinces him to see the world through a different lens. Or he could be hardened and damaged by his military experience and Wonder Woman seeks to help him find the hero within himself once again (i.e. should could act as a source of inspiration).
I don't think he needs to exhibit any kind of sexist behaviour to force Wonder Woman to teach him a lesson about equality. I think he can make the standard kind of comments that one would when meeting and travelling with an exotic demi-goddess warrior princess, but I think he should still treat her respectfully and don't disparage, discourage or dismiss Wonder Woman in her heroic endeavours.