With The Feast/The Fold, they need to all be upped to Level 70 and only be playable by people who have reached the Level cap.

With Frontlines, I think it would be perfectly fine if there was a level scaling system implemented to allow for more people to participate.

Feast/Fold requires people to know what they're doing because each individual has more of an impact on each match. Frontlines can have as many as 24 people per side with three sides. There's plenty of room for lower level toons to hop into that.