Oh, that's fine for an independent title (and I think someone has already done something along the lines of 'Dallas through the air ducts for as long as can be survivable'), but something like this is going to have a proper budget allocated to it, which means trying to think up lots of content filler. If you just want a bare bones concept, there isn't very much extra you can realistically put in there. This isn't the kind of thing where you're trapped in a maze and need to out-run a timer. It's going to have an attempt at a story, characterisation, different levels, maps and so on: Objective-based. If you're doing nothing but trying to evade an Alien, over and over and over and again, I'm not sure that's going to retain many peoples' interest. Not in the kind of numbers this will need for sales.
Besides, if it was a simplistic formula, it would have been released by now (assuming this is one and the same as creative Assembly have been doing) - or we would have at least been given a lot more publicity about it.
I suspect that people need to start picturing this as a generic shoot-'em-up with an Alien playing the part of a serial killer in the background.