Just my opinion but we're nearly 25 years into professionalism and clubs forming the tier below international rugby in England so trying to force in a level in between wouldn't work. Fans wouldn't buy it.
The solution is for all stakeholders in European rugby to work together, not just the RFU and PRL but the IRFU, SRU, WRU, FIR, FFR and LNR. as equals. Cut the amount of games played. Unions agree to compensate clubs for any potential shortfall in revenue by doing this. Combine the leagues under one umbrella. Get rid of promotion and relegation. Strategically plan expansion to new territories like the NRL, AFL or NFL. That's how to grow the game while looking after player welfare.
Squads are so big because there are too many games. Fewer games equals smaller squads and the star players play a higher percentage of games. Stars playing keeps broadcasters happy and leads to larger commercial contracts. Fans are happy because they too get to see the stars play and turn up more often.
England have a conveyor belt of talent coming through u20 level for the last number of years. Why aren't they getting a shot earlier?