Last I heard (and fairly recently), there is still a dire lack of knowledge club-side as to what replaces the Holden Cup in 2018. Tentatively it should be for all intent and purposes a state-based U/20s competition which will likely still be the lead-in game for NRL fixtures. No clue on how the cap will be structured for said competition but there has been some talk of 5 developmental slots given to each club in addition to a Top 30 for the NRL cap, so I guess that would play into what ever system is put in place for the U/20s.
Given how painfully slow the NRL has been in setting the 2018 cap, I am not surprised in the least that there wouldn't be a definitive plan already set for the third most important grade in Australian and New Zealand Rugby League.