No one seems to have mentioned this: I don't know whether it was during the returning champions era, but I definitely recall a week where they had a mix of regular contestants playing for themselves and soap opera stars playing for charity.
I won't call for a Sky's the Limit change, but I would like to see them go back to returning champions. (I like returning champions for their own sake, but forcing them to air shows in order would certainly be a bonus.) When the top cash prize was $25,000, and the limit was three days, my idea was that on the third day, you should play for whichever prize you want among those that remain. As is, I'd give a retiring champion a choice of spinning or picking one non-cash prize for his bonus.
I like the toss-up puzzle in principle, but only before the first and fourth puzzles.
There are two variations I've always thought would be good on the bonus puzzles themselves: 1) Have puzzles where there are exactly three different consonants and one vowel not showing so that, in theory, everyone has the same chance of calling the right letters. 2) Reveal fewer letters, but reveal each letter before the next one is chosen.