For daemons I figure the most likely option is they'll elaborate on the brutes and do up a set of undivided lesser daemons, beasts, herald/prince etc. in that sort of style. Could probably get enough unit variety for an additional strategic asset type out of just one infantry sprue, and leaves the door open to then do more later for daemon engines, god specific sprues and so on to flesh it out into a full army.
There's also the possibility of a mixed sprue with a handful of each but I'd hope even GW would realise that'd be really awkward, and annoying for a lot of the target audience.