Here's one that came to mind for me recently: Eat the Meat!
Using monster meat to affect monster evolution in the SaGa 1-2 games. (3 has a much saner system.)
Basically there are 3 families of monster across various power levels, except it isn't linear and you can absolutely downgrade yourself by eating a level 1 monster's meat.
The 3 families thing apples to SaGa 2, not to 1.
Also, since the game lets you choose your party, you can simply choose to put no monsters in your party. In fact, making that choice is certainly reasonable, as they tend to be the weakest race in the long term, with the only good forms being those with magic (which is still weaker than the spells you'd get from a spellbook) or HP drain attacks.
Worth noting that, in SaGa 2, humans and espers have terrible stat growth for most of the game, while robots are terrible at magic. (In the remake, a robot can effectively be a caster, but you need to find and equip magic circuitry to get non-zero magic power, and you still need to actually equip the spells you want to use; you get 7 slots, so you need to distribute them wisely, and won't have room for speed or defense.)
Speaking of the SaGa 2 remake, one change is that eating meat can't cause you to lose more than 1 rank, or go more than 1 rank below the enemy. Another change is the addition of the ability to keep a spell when changing form, and there's also mutations, which give you a monster with better skills (including, for example, the ability to heal the entire party).
As for SaGa 3:
* For the original, there's the issue that you can permanently miss a form if you level up too high, and the level 31+ forms aren't necessary the best. (The level 31+ robot is the best robot form, but that's not true of the other races that can transform.)
* In the remake, monster transformation is a bit more like SaGa 1 or 2, except that you can't lose more than 1 rank when you eat meat, or go more than 1 rank below the enemy's rank; plus, you can gain ranks by eating EX rank (which can break the early and mid game, but feels balanced for late game when it's easy).
you can absolutely downgrade yourself by eating a level 1 monster's meat.
In the SaGa 2 and 3 remakes, this downgrading is limited to a max of 1 rank of loss, so your sprite isn't going to become a weak little snake if you eat the wrong meal (instead, there will be no transformation).