Of course there could be a chance with large gulls of some element of hybridisation - the taxonomy of these is still not really clear. But gulls are also very variable and some ID criteria are also unclear.
Based on observations of Hong Kong birds, I don't think you would ever see a Heuglin's looking quite this pale (even in strong sunlight after bleaching). I agree with Bart that this appears to be a mongolicus and I think the features are probably within the range of variation for this species.
