If the NRL introduced the ability to roll over unused cap then shit-to-mid tier teams might finally have a mechanism beyond TPAs to compete for genuine super stars without completely crippling the balance of their roster. Newcastle, for example, with all their ample cap space left for next year wouldn't have to throw it at whatever dross is left on the market and could stock pile a true war chest for the 2019 player market.
For reference, franchises in the NFL are required to spend 95% of their cap over a four-year period. This prevents constant hoarding of funds but allows (in conjunction with the draft to be fair) the rubbish clubs (except Cleveland, who are a special kind of bad) to rebuild relatively quickly given good administration.
It wouldn't work 1-to-1 in the NRL given that most clubs here are not the financial powerhouses of their American cousins but introducing a modest salary cap floor and the ability to roll spare cap space over could have some merit here.
The NFL model is vastly more complex than the NRL with stuff like compensatory picks, guaranteed/unguaranteed salaries (a hugely unhealthy thing for players mind you), trades and the draft. Of all that the other thing that might have interesting avenues to explore is a NRL equivalent to a compensatory pick but that is something for another post.