I thought it would have been a good idea to assign someone as a right-hand to keep track of tags. But then...wouldn't they have to read every RP and then tag accordingly? That would be such a pain!

As for assigning them as pre-genned like...you know on YouTube when it gives you suggestions in buttons when tagging a video you upload? Like, just under the "enter your tags here" bar, there are like little suggestions according to the title of the video (in this case, title of RP). So if my RP was called "Fall of the Ninja Empire" (Using your example, @Code Guru!) then the generated tags would be "fall", "empire", "ninja" and then words that might relate.
Does this even make sense to anyone? Someone needs to upload a video and come and confirm if we can even do something like that.
And I'm really looking forward to this whole tag system.
And I think "casual", "free" and "advanced" should be tags too. Depends, if you're planning to run the search on every thread on the forum ever, or just within particular sections. The former would be better as then you could filter out the kind of RP in NRP, Tabletop and 1x1 that you were fit to play in, as ShienVien stated.