I was all for that theory, then kinda went off it, now I'm back on it lol.
If this is true, then we have to look at their contemporary positions:
- MIB is (seemingly) the head of a very important foundation of some sort. One of the guests tries to thank him for helping, the Westworld staff treat him almost as a VIP and give him free rein to do anything he wants, and he obviously has huge wealth if he has been coming to the park for 30 years at a cost of $40,000 per day - just for recreation. He is presented as a cold, experienced and calculating individual.
- William is almost the polar opposite. We learn he's climbed up a corporate ladder somewhat, but is still in an executive position - yet Logan scoffs at this and states that William will 'never be a threat to anybody'. There's also a reference to his 'cheap black suit' which could mean nothing, or be an indication of a particular dress style William favours - just like the MIB. William seems almost naive, doesn't like abusing the hosts, and is presented as an all round fairly harmless individual. But as Ep5 progresses, we see him start to change slightly and respond to the freedom he can indulge in within the park.
Could William become MIB? I don't see why not. Something traumatic is going to happen him I suspect, which will affect his view on the world, and this epiphany may drive his subsequent change into the more dangerous individual that is the MIB. With 30+ years of rooting/tooting/shooting in the park, he could become anyone. I rewatched some of Ep5 there and something else stuck out - when Lawrence calls him a 'sadistic f**k', he says "there's not a man in the world would take the tone with me you do ........................ in a past life, perhaps..."
I've a theory that if this is true, William may end up killing Logan - for some reason - perhaps blaming it on the park. And the development of this killer instinct in him could drive his hunger and his progression from an Executive VP role in his real world job to overall CEO. People don't become CEO's by being nice.