Batman is the big draw in the DC Universe with he and his associated characters claiming ten of the starting New 52 comics (Superman second with just four) and currently sitting at fourteen, including a weekly series, and two more on the way. Note that this is him and his associated characters - the Robins, Batgirl, Batwoman, Batwing spun off from Batman Inc., Catwoman, Harley Quinn. With the exception of the villain books (and the forthcoming teen mystery series Gotham Academy) they’re at least peripherally members of the “Bat-family” - sidekicks, counterparts and associates.
Would you want to play something like that? A central PC and associates rather than a disparate team of diverse origin and equal power? They can be compelling characters in their own right, but how they relate to the “star” PC is always a factor.
Some characters naturally attach and interconnect - a Vampire Slayer and a Watcher, for example, and even then one isn’t essential to the other. I most often see this in licensed settings where that model’s already been established. For example, the option to play a vampire and retinue of ghouls has been suggested in Vampire but I’ve never seen anyone do it, indeed I’ve only seen a single PC Kindred and PC ghoul together on a few occasions. Sometimes playing the sidekick is interesting, but I wouldn’t want to have that status by default.
An NPC mentor is different, of course - they might bring the PCs together and be the questgiver, but as NPCs they naturally fade into the background. A PC mentor is always there... and one who is an active team leader as well could double the issue of spotlight hogging.
It’s something I’d consider... but only with certain players in the leading role, just like I wouldn’t assign the Captain’s chair randomly in Star Trek. It also suggests group character generation... which is often a good idea but does not work for everyone. In this case, player 2 might have to create a character who could conceivably work with player 1 and fit their style as well, and that puts player 1 in a powerful position.
The other mentioned option is a team of otherwise unconnected characters. This is more normal, and is the case in just about every superhero game I’ve played in, with some limitations (which one player will usually fall outside, deliberately or accidentally) in some cases.
Somewhere in the middle, we have not-exactly-fan-favourites The Outsiders, a bunch of otherwise unconnected characters who work together with Batman as their mentor and sometime-absentee leader because... uh... is that the time I must be going. None of them count as Bat-family characters (except Katana taking the place of Robin in the recent Beware The Batman animated series, with her mystical origin nearly entirely dropped) so they don’t owe anything to Batman in style or name, just work with/for him. This is the usual way to deal with a more powerful “leader” PC (like a Vampire Slayer with a bunch of Scoobies) - it loses the branding but that’s not necessarily a bad thing.