This is how I see it, If you trying have it completely make sense it won't ,Civil War was to make money by having two side heroes fight.Marvel heroes wouldn't fight just as much DC heroes would and Captian America taking postion that he did make any sense.So with that in mind this how Civil War would have went with DC
Who would have been the major players?
Lex Luthor,Batman,Superman Pro Reg Heroes Aquaman,Wonder Woman,Mr Terrfic,GL John Stewart Anti Reg Heroes Green Arrow,Flash,GL Hal Jordan,Bat family characters
Who would taken the Iron Man role, advocating for a registration act?
Lex Luthor would take the Iron Man role,It just makes more sense replace everything Tony Stark did Civil War feels like what Lex would have done.Of course you would have to build up Lex legit hero before the event but if you are stealing Civil War then just picture lex replacing Norm Orsborn in a DC version of Secret Invasion(yes i know it came after CW) where White Martian try to take over and you have why he is considered a hero.
Who would have taken the Captain America role, denouncing said act leading to their (temporary) public murder?
Batman would have taken the Cap role.Of Course He is the hero that cares the most about keeping his idenity secret.Batman is interesting because he would understand the need for it but does think it would be the right plan and of course is proven right over the course of the story that exact plan of registration was wrong.
And who would have taken the Spider-Man role, revealing their identity?
Here is the big difference .The Spiderman role would have gone to Superman the purpose of story is about impact(and sales) and Superman revealing his secret idenity would be the biggest impact like Spiderman for Marvel.Of Course Superman working with Lex is hard thing to sell to comic fans but Superman just following the laws of nation makes sense and that how it would work.And based on the current Superman idenity revel I think DC fans would much prefer a press conference at the Daily Planet where Superman goes "I AM CLARK KENT" in the context of a huge story and watching Lois Lane,Perry White,and Jimmy react in that setting would be great.
What things would likely have gone the same way?What things would likely have gone differently?What would have been the likely fallout for the DCU?
More less Civil War it set up to have heroes fight, So it goes about the same Superman realize at some point that he is on the wrong side switches side and since Lex is villian instead of Ironman he does something wrong (probably kill or lead to Superman death) which cause all the heroes to unite against him.Fallout is the heroes not trust each other for awhile.Civil War where the heroes act in completely in character is just angry convo between heroes where at the end they go and beat Supervillians together for Civil war to work you need characters to act out of characters at some point.