We had this talk in BUF before too and I think I brought it up in ANC here and there. It was prior to the expansion that brought in NL and ANA. Basically what we thought was the new middle league would need some sort of stable environment so there isn't too much inconsistencies and roster turnover.
We thought back then that in the SMJHL only SMJHL rookies and freshly draft SHL players would be able to play. ( The freshly drafted SHL players would be capped either at 350 or 425)
The new AHL type league would consist of 4 teams (1 per SHL division and also pull their players from these divisions) So you have the Great Lakes team, Atlantic team, Northwest and Southwest team.
The AHL type league would be uncapped like already suggested in the OP and mostly be a place for developing players so they aren't being forced to be capped and the roster would be filled with IA free agents that aren't used. (SMJHL would only get FAs of 350 TPE and lower to fill up the bottom 2 lines and is forced to always play an active player in the top 6 lines above IAs)
If there is someone who goes inactive while being capped in the SMJHL but surpasses the TPE limit of 350 they could be used to an uncapped build in the AHL type league.
Obviously this idea was thought of before the recent SHL expansion which got us to not equal numbers of teams residing in each division.
For an AHL type of league to succeed you 100% need it to make it as stable as possible and the SMJHL would go back to their original purposes of being solely a developmental league with mostly SMJHL rookies and a few capped players + IA under 351 TPE.