When Insay Batman's potential, I truly believe that the next Batman movie could become within the top 20 or even top 10 highest grossing movies of all time. How many other DC characters can really say that?
I don't disagree with that, but I think you're overestimating the appeal of the peripheral Bat-family characters. I agree with your point that they are better off doing both, not one or the other.
Red Robin who I mentioned earlier, for example, would not have mass appeal.
Nightwing will have more, but it will still be an uphill battle, I suspect.
So many of the Bat-family characters are just "Batman-like" to the mainstream audience that I think there's both the risk that they feel derivative and feel like less important movies in general. Batgirl might be able to escape that as she's more established and other than Wonder Woman we still have not seen many women in these movies.
If I were tasked with charting a Bat-universe I think the most I would recommend would be:
Batman - obvious
Nightwing - with a hard focus on him being charming and trying to live a normal life
Batgirl - With a much lighter tone
Batwoman - supernatural horror
Gotham City Sirens - focusing on the camaraderie of some of the villains
That would be it though. I wouldn't try to go deeper as I don't think people want to see a Huntress movie or a Red Robin movie or a Matches Malone on a single mission, or a Ra's Al Ghul origin, or Elseworlds about The Mad Hatter.
The wider DC Universe offers so much potential for DC which has not really been touched yet:
Magic (Zatanna, or JLD if it ever happens), space (GL, Invasion), kid-friendly (Shazam, Elongated Man, Superhero Girls, Booster and Beetle), other teams than JL (Teen Titans, JSA, LSH), mystery (Plastic Man, Elongated Man, the Question).