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To justify adding another stage to a line (one that's "different enough"), it should have different body structures (while not everything can add another set of limbs like Charizard, a change in proportions is better than directly scaling up the body), a clear new development in the design elements that are already there rather than just adding more elements (for example, the Sewaddle line focuses on the leaf clothing motif and utilizes it completely differently in each stage) and some changes to the color focus (the more drastic the physical changes, the less drastic the color changes, so they're still recognizable - for example, Wimpod to Golisopod being the same colors is fine, while Mienfoo to Mienshao completely changing the colors is better because it's an obvious change all around where the two are more similar).
A good example of a line that does all three of these without being over the top or deviating too much from their basisis the Lillipup line. Lillipup is small and thin, Herdier gets mostly longer and Stoutland gets mostly wider, so every stage has different proportions; all of them expand on the facial hair in different ways, with Lillipup's whole face being messy fluff, Herdier having a short, clean moustache and Stoutland having a long, flowing one with a different design; and the color focus is different even though they all have the same colors, with Lillipup being more tan, Herdier being more blue and Stoutland adding another shade of each to keep it varied. They're a model example of an evolution line - they're not "just getting bigger," but they're also not so over-the-top "different" as to seem random.
I'm a huge fan of lines like Litwick or Caterpie where every stage is different but coherent (obviously "caterpillar-cocoon-butterfly" makes more sense than "increasingly mature caterpillar"), not every animal/basis has a line like that that works, so Lillipup is a fine example of doing it right without needing anything of the sort.
