So the series will be set a few years after the series finale. I'm guessing this means they will probably re-cast Herschel jr. as a teen and possibly feature Negan's kid. If they're in it it seems. Kind of wonder if it'll feature jerk Herschel from the comics finale. Also worth noting that the alternate ending for TWD-which would have Judith, Coco, Rick Jr. etc as young adults didn't feature seem to feature Herschel.....
As for Maggie and Negan, Maggie did have it out for him early in season 9 (But relented when seeing how pathetic he'd become in the prison) and the two still really hated each other going until season 11, and even in the finale she can't bring herself to forgive him (Because seeing him reminds him of Glenn's death) despite Negan's attempts to get through to her as he's been softened somewhat over the years partially due to Rick and later Judith's influence (It was Carl's dying wish for Negan to be redeemed). Herschel, on the other hand, might come after him at some point, although it's unclear whether this will happen in Dead City.
Some of the promo pics show Maggie with a knife to Negan's throat. So they're definitely not a couple but have a sort of tenuous alliance at best.