Balance is vitally important for making "From Bear to Phoenix" tactically challenging. So the cool ideas that I have for spells, or classes of mage, or whatever, ideally would be carefully reviewed and play-tested in case they would unbalance the game. (I guess there's an alternative - "throw them all in together and see what people think" :)
For example, I would like to give a Shapeshifter one or more "emergency" Forms that they could shift into if a battle was going badly. I thought of two such: Tree and Rock. Of these, a Tree would offer rapid regeneration of HP and MP (due to sap and photosynthesis), but would be vulnerable to most attacks, in particular blade and fire. A Rock would also regenerate HP and MP, both somewhat slower (I claim that a Rock is naturally slower than a Tree), and would be highly resistant to most attacks except blunt attacks (possibly also lightning and a few others). Both would probably cost 0 MP to change to; this is part of the nature of emergency Forms, that they can be selected if the Shapeshifter has low MP. Both would not be able to move or attack. Both might also have some Stealth: for example, opponents would still see a Tree, but might not be able to tell that it is a Shapeshifter in the Form of a Tree rather than a natural tree.
The question is, what if the opponents don't have any blunt weapons or attacks? The Shapeshifter could be unbeatable in such a battle: it could attack, change into a Rock when damaged, regenerate while ignoring attacks, change back into an offensive Form when it has regained HP and MP, and repeat as necessary. Trees don't offer the same problem, since they are vulnerable to most attacks (in fact, it might still be risky to turn into a Tree if you don't have allies nearby to cover for you).
There are a few possible solutions that I've come up with so far: having a non-zero MP cost to becoming or remaining a Rock (a cost per turn would make Rocks much less attractive as emergency Forms), or limited or no (MP?) regeneration while in Rock Form (in which case they'd only be useful for prolonging battles, or perhaps stalling while allies regroup).
Maybe the best approach is just to try out possible solutions: to implement Rocks on an "experimental" branch and play-test them to see how unbalancing they really are before integrating them into the trunk. It would be a shame if a Shapeshifter couldn't turn into a Rock at all ...