I have to say that Hollowfied Shinigami with Resurreccións and Resurrección: Segunda Etapa are both concepts that have potential. However, both were introduced with little explanation as to their true nature, both were implied to be exclusive to the single characters who used them, and both were completely forgotten after their respective users deaths. Plus with current events (and the fact that Shinigami/Hollow hybrids of any kind are old news) it looks like it is unlikely they will be further explored. Such a waste. The possibilites are endless. New forms and abilities for both sides. Plus the battles in the Fake Karakura Town would have been a lot more interesting I'll say that much.
The specific reasons I would've liked to see them is:
1. A Hollowfied Shinigami with a Hollow Form just seems like the best example of them mastering their Hollow powers. They have masks so why not have everything else? Lol. Plus it's technically a sword release so it fits.
2. An Arrancar with 2 releases would've increased the parallels between Arrancar and Shinigami which would make them more of a Hollow Equivalent to Shinigami in my eyes. Kubo could have even played with the concept of Resurrección: Segunda Etapa even further and made unlocking it one of the requirements for an Arrancar to become an Espada, which would make another nice little parallel between the Espada and Captains.