wth 7 elements? i think ppl would have only 3 elements, thats enough
2 elements to make combo
1 element to counter 1 of the opponents elements
this is the minium b/c u could have 2 elements that win vs the opponents 2 elements.
then:
Pein
Kakuzu -->exceptions
Kakashi-->exception, could use all 5 elements but he had to copy the enemis jutsus during the battles, of course in other battle this would be reseted
then for what Rei said about costum elements, i think thats np. Look at Bisha's lava, is earth+fire so u need a combo to beat just 1 attack of him (i think) amd costum element users have the advantage because they choose the weaknesses of the element.
and for this to dont be so restricted ppl make Bio's, thats why they exist! a bio with fire, wind, earth elements, and another with lightning, water and wind elements.
and then we could make the exams to pass to the next rank, and in the Kaga rank u could use the 4th element .
im just throwing some random ideas