I have two possible ideas.
The first, more developed, and probably more simple is, something with branching evolution possibilities; whatever form you were before would effect the next ones. Starting form leading to the first three forms is unchanged, but from there... your next form would still show the parts from whatever you were before, as a merged thing. So if, for example, you started with the red Brutality one and became the corresponding dragon, continuing to focus on Brutality until you evolve would result in the bigger dragon like before, but if you were to switch to Alacrity you'd get a form like a cross halfway between the red and green forms (yellow perhaps), or if you were to go to Mysticism you'd get a form in between those instead (magenta). For ease of art, you could get these combined forms from the opposite direction as well; red->green and green->red result in the same thing; only 3 new forms from that. Once you are the final stage form, you would not change forms anymore; no going blue+green cyan form and then suddenly becoming the red one instead. The only way to see the other forms would be via using one of the stat reset items and then going along a different path (and possibly by doing something after completing the game?).
The alternate, probably more complicated idea I had would be to evolve upon reaching specific stat totals, rather than a value with any one stat, and the form you'd get would be based upon the ratio of the stats at the time of evolution. If your Brutality is a lot higher than your Alacrity and Mysticism that would result in the dragon again, but if the Alacrity is relatively close then there'd be some trait from the green path noticeably mixed in, and if all three stats were relatively balanced then you'd get some sort of balanced form combining all three paths. This... would result in a lot more forms, and might require an actual overhaul of how mutations are gained, so whether you want to go for this depends on how ambitious you are and how much you are willing to risk breaking things that already work.
Regardless of what you do, even if you don't go for either of these ideas, I really do strongly recommend some sort of indicator for when you'll evolve. It could be a whole new piece of interface, it could be a little graphic tacked on to the existing interface, it could just be a little number saying that you will evolve upon hitting this value, there's all kinds of ways to do it.