That is a good question. Can I say lots?
Truthfully I don’t think we can begin to fix what’s broken without essentially rebooting the entire universe again, but I do think one thing that can be improved upon is the people working at DC understanding the value of their characters and the emotional attachments people form with these characters. Even if characters like Cassandra Cain and Stephanie Brown are not as well known as Barbara Gordon outside of mainstream comic fandom, they still have a fanbase and that’s still more money in DC’s pocket.
Similarly, I do think that if a writer has great ideas for a great story involving a particular character, I think that writer should be allowed to use that character and should be allowed to tell the story they want to tell. I think editorial should only step in if an idea a writer wants to convey is potentially (if not outright) offensive or if an artist depicts characters in ways that are exploitive. But that’s only in an ideal world, I suppose.
I get that comics have to constantly reinvent themselves in order to sustain and even gain a new audience, but I also feel that characters and stories should be retold in ways that actually make sense for their characters. For example you can’t have Superman without one of the most influential women in his life, Lois Lane, you can’t tell Wonder Woman’s story if it doesn’t break away from any sexist and misogynist ideas and stereotypes about gender, you cannot have Catwoman without Batman as an important influence in her life, and you cannot tell stories of Earth-2’s Huntress and Power Girl without seeing how they relate to their families and how their upbringing has moulded them into the people they become as adults.
On the subject of Earth-2, I think DC missed a great opportunity to do something great with this. If Lois and Clark couldn’t be married on the main DCU Earth because they are at the beginning of their journeys and need time to build a relationship, why not establish a version of them as a married couple on Earth-2 like they were originally? Similarly if they needed to undue Bruce and Selina’s relationship because DC has other plans for them (i.e., more focus on Bruce and Talia’s relationship to Damian), then why not keep them as a happily married couple on Earth-2 as has been reestablished? Or if Barbara Gordon had to go back to being Batgirl on the main DCU Earth to gain television audiences, why not have a version of the character on Earth-2 who continued to function as Oracle and was paralysed by other circumstances (i.e., the Apokolips War for example)?
Given that Earth-2 did have a first generation of heroes and the original Black Canary (Dinah Drake) was originally an Earth-2 character, I didn’t see it necessary to retcon Baby Dinah out of continuity. I liked that Dinah Drake was both a mother, a wife, and a heroine, and I liked that her daughter continued her mother’s legacy. Similarly, since the mainstream universe already has a Supergirl and Helena Wayne functions best on Earth-2, I didn’t see it necessary to continue a tradition of retconning Helena’s origin every reboot by removing Helena Bertinelli out of existance either, as was done with Helena Wayne in 1986-1989. I also felt it was an unnecessary development as it did not add anything significant to Helena Wayne’s story other than give her an alias for the main DCU Earth. Being Catwoman’s daughter alone should have made it easy for Helena Wayne to come up with any name she wanted and falsify the necessary documents.
One of the great advantages of having a parallel universe (in my opinion) is that it allows for a different kind of storytelling, and should give you an alternative to the mainstream universe. It’s also one of the few ways you can try to please everyone or “have your cake and eat it” as they say. It works perfectly well at Marvel, I’m not sure why it couldn’t work out at DC as well.
Truthfully, the only thing I liked out of the New 52 was the reinstating of Earth-2 and reestablishing Helena Wayne and her friendship with Power Girl, but even these could’ve been handled a lot better than they actually were.