The problem with that is that it forces people to write a certain type of story, and they might not want to write that particular type of story. And, it's difficult to design a premise that is fair to everyone (i.e. not excluding certain characters, settings or writing styles). Also, it likely means we'll get a large number of similar fics. I'm not keen on the idea overall, but if someone can suggest a good premise then it might work.
I had an idea for how to organise the contest. Basically, my plan was for the contest to have three categories, which would be porn, lemon with plot and non-lemon. However, rather than entering a fic into one of the three categories, a fic is entered into and judged in all of them unless the author requests otherwise. That allows people to write PWP, lemons with a plot or non-porn without having to subjectively compare a porn fic to a non-porn fic. Of course, each category would have different judging rules (in particular, the porn section would be judged on hotness and not on plot, whereas the non-lemon stuff wouldn't be judged on hotness at all), so the fics would get different scores for all three categories. Their overall score would be the highest of the three.
Also, to counter the problem the BL judges had, I'd add the possibility of judges to remove points outside of the designated scoring system for things like really awful presentation (where the writer has clearly not bothered with beta-reading), horrible OOCness (to the point that it's an "in-name only" fic) and, in particular, an otherwise good fic that is only tenuously related to the Nasuverse, if at all. Thatmeans that you won't need to set up the scoring system such that any remotely well-written fic will score at least 50 even if the plot, characterisation etc. are terrible (in particular, the "technical" aspect wouldn't need so many points allocated to it, because a fic that was clearly not up to standard would lose points for it) and it would also give the judges more discression to handle situations where the author really should lose points, but they can't find a good category for doing so.