I was thinking on similar terms but, there are atleast couple of factors to consider;
1) You don't want to decrease the quality of your domestic competition especially when it comes to red ball cricket
2) Can be tried in white ball cricket but, the thing is it would be harsh on regions to get their emerging players from them which they have developed through different competitions. Argument can be about players who are not selected for main squad but, then again wouldn't it be better for them to play 2nd XI alongside some experience player to learn further? There would be limited to no players who are neither part of any U19 teams of the regions, 2nd XI or main teams and that would make it difficult to form a team every year.
So these are couple of factors to take into account. WI used to have an emerging players team in their 50 overs competition as you are suggesting but, I guess that's not there anymore.