So after rulebook is published for example, teams have until week 13 to "Enter competition" (manual action, needs to be done EACH season - EMAIL and big notification on website maybe) this way game knows how many people are active. Based on that if X=200managers suscribes there will be nbDivs=pair(X-12/12)=16Div2&3.
In current system we want 4 relegated teams max :
for i in range (4, 1,-1) ; if nbDivs%i == 0 : Divs2 = i
16/4 == 0 so 4Divs2
Divs3 = nbDivs - Divs2 = 12Divs3
This method works when X<253 ofc. (if it's not clear I can give u excel version).
Looking at suppliers theres actually approx 126 active managers, with my system this means : 1Div1, 2Div2, 8Div3 (less but full).
Another idea is to forbid creating teams before new rule book is published : they cannot enter current competition only prepare their cars for next season... maybe "encourage" new managers to take an available job.
Also it could be funny to force teams to have their deals signed with supliers for next season before allowing them in.