To be honest, i think HAAS is very smart in not wanting to become some sort of 'Ferrari slave', which would also be negative for their name.
I think a Haas - Chrysler would be cool, but i'm sure Ferrari would want some finger in the mix there and then.
Haas is taking supplies from Ferrari already, and did so to build their team. It's now a case of learning and becoming able to do things 'on their own', which is a much smarter move.
As such, they won't be 'Ferrari slaves' which means they can decide for themselves which sponsors to sign, which drivers to sign, which personel to sign, and not having to let a Ferrari-powered car (like an Alfa Romeo Sauber) just because they have a Ferrari partnership. They also don't have to do Ferrari-forced PR stuff, and be all that they can and be all by themselves instead.
That also means they are free to choose what to do at any moment in time. They could sign for a Honda engine for 2019 for example. A Mercedes engine. Or perhaps Cosworth does step in somehow, perhaps even through Ford, who knows. They got some critisism recently for stating there are no 'capable' F1 drivers from the States right now, and though it politically sounds a bit painful, they're just darn right about it.
In any case, Haas is one of the few teams in F1 that is not forced in any way to do something they don't want or could decide for themselves. Matter of fact, they might be the only team in the grid that can do whatever they want because their funding and sponsorship comes from themselves, from Haas. Not from some corporate head like Marchionne. Not from a contracted sponsor like BWT demanding their livery. Also not becoming a forced 'gateway' to dump junior driver programme participators in their midst, like which will happen with Alfa Romeo and happens with Toro Rosso.
I know this is tad offtopic as it's not the Haas thread, but since they were mentioned here, it fits.