Well, it's on the panelists to be prepared and such. The fields you have to fill out are pretty comprehensive, too, so just writing an idea for a panel is something I think wouldn't cut it for getting approved over some of the more well thought out submissions. If I recall, there was a bit of a line on the wait list last year, so FanimeCon isn't hurting for panel submissions. Basically, I would suspect with FanimeCon's growing popularity, the panel submissions will increase and force everyone to submit their best stuff to get in, and also give their best during the panel so the staff are more likely to accept them back when they look those panelists up the next year.
Ultimately, it's just an application, and like any resume, CV, or application, you can make it look as good as you want (like a video submission), but it's how that person will perform that matters. You have to look for clues in what you have - did they take the time to use proper grammar and spelling, did they articulate their points well, etc. But again, people can put on an act just to get in the door, so raising the bar for what people need to submit could weed out those not serious, but also be limiting in some unforeseen way at the same time.
Again, in the end, it's up the panelist perform on their end come con time. Feedback is also an important part, and I know FanimeCon wants to eventually offer panelists a way to see their feedback. That would greatly help everyone so they could see the criticism and take that into consideration for future panels.