Time for the Quiz Demon himself to chime in!
1. and 2. As SweetDalilah said above, I have several new quizzes in the works, but because of the recent quiz fever (and other reasons), I won't be posting my next one anytime soon, so I will look forward to chrissie's quiz in the meantime (and those from any other budding quizmasters and quizmistresses!
) Yes, quiz themes and quiz sizes are limited only by the quizmaker's creativity and resourcefulness, and not necessarily by the size of his/her game collection! The wide range of games usually allows for quiz 'sequels', as mentioned by diego (there were two phone quizzes, three map quizzes and four quizzes from my tree series). So far, no one else has been insane enough to create another superquiz (100+ pics, by my definition).
For some weird reason, each time I announced a new superquiz, people would start drooling! 
3. and 4. The pros and cons of guessing rounds vs. PMs have already been weighed, so there's not much more that I can add. In my earlier tree quiz series, I offered the PM option to anyone who felt confident they recognized a majority of the pics and didn't want to be limited to the maximum number of guesses per round. Only a handful of people ever chose this option, so I discontinued it.
However, after looking at the results from last few quizzes, I'm thinking of reinstating the PM option for the "quiz hogs" and "know-it-alls" who want to show off their prodigious adventure game recognition and memory skills
, and to give more people a chance to participate.
