Robert Bowling has already confirmed that the way killstreaks work has been completely revamped. In the short clip at the end of the Survival Mode trailer, it shows him using 1 killstreak, getting him another "point" towards his next killstreak, meaning killstreaks now stack.

Original video:


If you take a look at this first image, you see that 13 ticks filled in. Once he gets a kill with his first killstreak, he gets the 14th tick which gives him the juggernaut killstreak.

Image of him before getting the kill:
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Image of him after getting the kill:
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As you can see, there were 14 ticks before+4 more after he got the juggernaut killstreak, meaning the highest killstreak is 18. Killstreaks also stack, as you see in the bottom corner, the image below the predator missile icon IS what kills fourzerotwo which gets him the juggernaut killstreak.

You can also choose which killstreaks to use first as you can see by using the up and down directions on the d-pad to select the killstreak and right on the d-pad to use the killstreak.

One can also assume that since they are called POINTstreaks, you can get "points" by getting kills OR playing the objective(capping flags, planting bombs).

Proof that it is multiplayer:
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Notice the points at the bottom left corner. If this were Spec-Ops, there would be currency.

TL;DR: Killstreak renamed to pointstreaks. Killstreaks stack like COD4,WAW,MW2. Can get points by getting kills or playing objectives. Can choose order of which killstreaks can be used. Highest killstreak is 18.

Thanks for reading guys! Hope I could shed some light on the topic. If you read ALL of it, I appreciate it a lot ;).

Please thank the topic if this helped you.