Expanding on my previous idea a little:
The Weyland Yutani executive would be a high-ranking officer sent to the Nostramo to oversee the operation. While not initially hostile, his goals usually put him in conflict with the Colonial Marines. His goals could range from "Secure 10 alien corpses", "Capture or kill all Nostramo survivors", "Secure a live alien", or "Ensure no witnesses to the infestation escape".
To accomplish his goals, he has a small team of PMC soldiers to directly command. He can present himself as hostile or friendly, and he can attempt to forcibly accomplish his goals or deceive the marines into achieving them for him.
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Absurd idea)
An experimental mechanic that could possibly be implemented would be his ability to bribe marines. Using a flash or an implant (using borrowed rev code), he could convert a few marines to his side. It could be explained that he offered them a large sum of money, has their family hostage, or brainwashed them. The crooked Marines, the PMC, and the Exec would all have WY flags (much like the Rev R flags and the Syndicate S flags) to identify eachother that are invisible to non-members.
The crooked marines follow direct orders from the Weyland Executive (in person or via PDA), but should never attack their fellow marines in doing so. They are, in fact, still Marines but with slightly questionable loyalties. They can feed him intel, ferry him supplies or equipment, transfer aliens caught in the field to him, or try and lead the marines away from important or sensitive areas. If the Marine reveals their hidden allegiance or they are found out, they may be forced to escape from the Sulaco and join the Weyland forces officially. Should this happen, the Marine *must* shed their uniform as soon as possible and adopt a Weyland-Yutani uniform.
All in all, this idea is basically modified Rev with the Revhead(Executive) having objectives instead of the usual rev goal of killing/converting everyone else. If he uses a flash to convert marines, it should have a 50% of breaking per flash. If he uses implants, he should only have three or so.
Of course, if this is all a bit too ambitious, the WY PMC could always be a replacement for the rebels.
