Wikimon has by far the best list of Digimon, as well as additional information. Lemmie supply you all with each list in case it's a little hard to find it yourself under the cut.
[hider=Digimon Stages VIA Wikimon]
Fresh http://wikimon.net/Category:Baby_I_Level
InTraining http://wikimon.net/Category:Baby_II_Level
Rookie http://wikimon.net/Category:Child_Level
Champion http://wikimon.net/Category:Adult_Level
Ultimate http://wikimon.net/Category:Perfect_Level
Mega http://wikimon.net/Category:Ultimate_Level
[/hider]

Plot related, if we're going with a simple premise like this, we should think up villains, one for each stage most likely. Champion is the first boss, Perfect is the second, Ultimate is the final and hardest.