While it might be that someone could still enter the existing contest, since the existing contest started about six months ago, and the close date is now only a month away it doesn't seem to me to be unfair to the organizer of the current competition to begin discussing the next one(s), expecially when the discussion is in general terms. But then, my position would be that it would not necessarily be unfair to have more than one competition going at once, as long as the each was significantly different from all the rest. For example, if one ongoing contest was the best reproduction of the Colorado and Southern Narrow gauge, with four months left to run, and another ongoing contest was the best representation of the a fictional short line in Australia inspired by the BHP ore train with eight months left to run in the contest, I don't think it would be either unfair or improper to open a one month contest for a "complete in 3 baseboards, due in one month" route inspired by railroading serving small coal mines in the geographic region of your choice. And while I would agree that it would be unfair, one month into an 8 month contest, for someone else to open a contest that was substantially the same, and though I'd think it reasonable and proper to ask permission first, I'm not sure it would unfair to start a substantially similar contest when the first was nearly over, but before it had formally closed.
Finally, for what it's worth, if the present discussion were overly critical of the person running the existing competition, or one of the judges, or some of the entrants or their work, then I would concur that it would be unfair, impolite, or downright rude.
ns