I would probably keep 3rds as its a natural starting place for Colts who need another year or so.
The general framework I have in mind (which I think I've put up before) is 10 clubs in each Division, split by conference if required for Division 6 which seems to need it:
PREMIER - 3 Grades + 2 Colts
FIRST DIV - 3 Grades + 2 Colts
SECOND DIV - 3 Grades + 1 Colts
THIRD DIV - 3 Grades + 1 Colts
FOURTH DIV - 3 Grades
FIFTH DIV - 2 Grades
SIXTH DIV - 1 Grade
i) Colts as listed are a requirement. No exceptions. Clubs need to find a way in order to make sure development pathways are available to school leavers in particular. If they can't, then they still have a competitive 3-grade competition in 4th Div.
ii) Masters is a separate competition. They can be part of a club, but can't contribute to club championships, and will be largely organised along the lines of Halligan Cup.
iii) Any leftover Colts are organised as size of competition permits, but NO single division/conference should exceed 10 Clubs. e.g. if there are 16 Colts teams left over, they will play in two conferences of 8, with an appropriate number of top-ranked teams from each conference playing in the finals e.g. top 2-4 depending on numbers.
iv) The big assumption I'm making is that lower division clubs are going to be fed by juniors as well, and that players reaching Colts age will assist 3rd Grade unless they play the pickup Colts comp. Clubs need to develop juniors in order to sustain rugby right through the life cycle.
v) Promotion and Relegation on Club Championship for ALL Divisions. With an equitable number of grades to field, it doesn't then become a stretch for a side to go up in playing quality AND numbers - my biggest beef with the current system.