Fire and Lightning are the strongest, most destructive elements...Wind is powerful but it cannot stand up to Fire or Lightning...
Earth would be the weakest.. in strength that is...but it's best for defense.
As for randomness, well...it basically is randomness...but keep in mind that the effects of any of the elements WILL make you stronger..not weaker...you will improve in some way if you get an element..
also keep in mind that the powers of the Amulets are not set(except for maybe the Fire or Lightning Amulets, just because their effects are perfect for their type..
)
So, the Earth amulet MIGHT increase defense...or it MIGHT give you another boost..
Anything can be changed, the BASIC concept is to implement Elemental Alignment into the game...
Whether or not it's permanent can be changed, the buffs for each element can be changed, etc. etc.
A price will most definetly be involved with this AS well as a minimum amount of Dragon Kills...
Meaning it's similar to the seal...EXCEPT you can be good, evil, or even neutral alignment..
As for a fire mastery person owning forest creatures...First off, most of the players who would be able to master Fire, can already own forest creatures...
As for trainees in Fire or Lightning..Well I could easily make it where you get hurt every round...or make it where you might missed on some of the flame/lightning attacks..
There are 100 ways this could be designed...100 ways it could be changed...
The basic idea, once again, is Elemental Alignment added to the game...That could be considered the real question...Should/Could/Can we add Elemental Alignment to the game?
By the way, in the manga, elemental alignment is a Jounin level skill...It takes years to master and mostly, only high level shinobis can achieve mastery...
Kakashi, who is ranked the highest level Jounin in Konoha, has not even mastered half of his element...He takes alot of strain just using the Chidori a few times..