Elaine being the main character for a game would not work, since she does not have the same charisma as Guybrush, and writing funny/interesting dialogue and commentary would be a whole lot harder for her. Guybrush is the perfect playable character, as he's basically a vehicle for funny jabs, comments and one-liners. It's the character of Guybrush which breathes a whole lot of life into the Monkey Island world. Doing things from Elaine's perspective would not be as fun, as interesting or as effective.
Elaine and Guybrush having children is a terrible idea, I think. For one thing, it's predictable and is an unimaginative direction to take the series. Also, it would restrict and tie down the series, to a degree, because it will bound two of the leading characters to a responsibility. And having the children be a 'mini-Guybrush' and a 'mini-Elaine' would just be incredibly corny and just downright inane.
Having Morgan as a definitive villain would not work either, as it would defy a lot of her actions in the later chapters of 'Tales of Monkey Island'.
Also, the best way to mix up the formula is simply to have a new villain. LeChuck's the reason why the series' main plot is becoming so repetitious. He's the reason why Elaine keeps getting kidnapped, he's the reason why Guybrush's main aim at the end of every game is to find a way to kill him (obviously), and he's the one who is constantly forcing the formula into a repetitive cycle. Yeah, we all love LeChuck, but it's time to keep him dead on this occasion, unless we all want to have the same core story and character motivation in the next game.
The series needs to be refreshed in some way to prevent things from becoming stale, but new playable characters, change of allegiances, and the introduction of children into the story - these are not the ways to do it.