I would like to suggest adding more stones and tweaking existing ones for the sake of diversity.
As of now, the only stones viable are speed, attack and health. Almost all other stones are deemed either special cases or completely useless.
Here are some suggestions, I will refer to the stones by their property.
Here is the current state:
Attack: multiplicative 5/10/15
Crit chance: additive 2/4/6
Crit power: multiplicative 5/10/15
Speed: multiplicative 5/10/15
Defense: multiplicative 5/10/15
Health: multiplicative 5/10/15
Crit chance: This stone is exceedingly underwhelming. It uses an additive approach which is great but the percentages are so low that it barely does anything for anyone. As of now, very few characters benefit from crit other than those that have crit related abilities (Shan Yu) or multiple strikers (Zurg or Wasabi). Even then, it is more beneficial to get Attack stones since the increase in damage is guaranteed.
I would recommend raising the chance to crit up to 5/10/15% to be on par with the other stones. This is still a chance to crit, not a guarantee.
Crit power: This stone is also very underwhelming. This is an increase in damage comparable to attack but only on the condition of a critical strike. This means that if you do not crit, you will never see this stone's effect. At this stage, even getting a health stone is more beneficial.
I would recommend raising the damage power to 7/14/20% at least if not more to make this stone somewhat differentiated from attack stones and bring a greater effect for the risks taken.
Speed: This has become the de-facto stone for everything in the game. It turns out that having faster turns and more turns is worth more than almost anything else.
The issue with this stone however is that it is multiplicative and too strong in values.
A fast character will only get faster and outpace everything else. A slow character will probably get slightly faster but cannot catch up.
I would recommend decreasing the benefits down to 2/4/6 as a static additive method. This will benefit slower characters more in the long run than faster ones. This will also encourage using different stones on faster characters.
Recovery: This stone is very lackluster as there has been a very careful approach being taken with recovery in this game. Most healing is very weak or useless. It basically trades a turn for maybe 1 more turn here and there but it is negligible.
I'd double this completely and see if healing does become an actual viable strategy. At least 10/20/30% more since it is already pretty low.
Defense/Attack/Health: Cannot comment on them, at this stage they are fine arbitrarily.
Now you'd think that's all folks, but it isn't.
Consider adding new stones to the game to improve it up a notch and create even more diversity:
Tenacity: Additive 5/10/15%
Precision: Additive 5/10/15%
Potency: Additive 5/10/15%
If possible, make precision potentially nullify evades. This stat is very lackluster otherwise.
Note: DO NOT EVER make an evasion stone. It looks fun and all but basically it will become the new defacto stone after speed to avoid taking damage altogether. Evasion is already not a fun mechanic and making it even more prominent will break the game.