I definitely think Wallace's mother, or at least her family, could provide a lot of rich narrative soil. Hell, even Daniel could be used to good effect. The guy was a dead end as a Flash villain but with a few tweaks he might still benefit Wallace. Even if he remains dead (and that might be for the best, really) I feel like there's *some* degree of potential in Daniel being used in Wallace's story.
I dunno about Wallace's mom being related to another prominent DC family. As a general rule I like to keep franchises independent of each other; it gets messy when there's too much interconnection and one franchise decides to retcon or reboot while the other doesn't (among other problems). And maybe I'm wrong here but wouldn't that invite a "we don't all know each other" kind of thing?