ISIS doesn’t have the physical numbers to occupy Baghdad, nor do I think they’d want to do anything but bypass it, concentrating on their dash to the Gulf. OTOH, if there’s support in Baghdad, it would make a nice buffer and anchor. ISIS found Kobane too costly to hold, but is starting another attack, they have a sophistication to their movements. Kobani was a bad gamble for them, they took serious losses, not least of which was their air of invincibility.
There is not much beyond what they have already that is still a Sunni region. I think and have heard they have embedded sleepers in Baghdad. Kind of obvious with the rash of prominent bombings there. generally the Tigris river divides the Sunni to the west and the Shia on the east bank. I do think ISIS would like to get into Baghdad. It would be a hell of a recruitment on the media for them. There are a number of Sunni tribes and some Sunni militias aligned with ISIS. I doubt it will remain so, but just using them to gain ground on the Shia. They may be more welcome than we would like to think in Western Sunni districts of Baghdad.