The major key to success for any large open-source project like this is a thriving community. If a community does form around this project, let's be sure to establish a supportive and welcoming tone from the beginning.
Diversity in particular remains an issue in the tech industry in general, and open-source especially - and a lack of diversity is difficult to correct after the fact. In building a community around this framework, let's try to draw a diverse set of developers in the beginning, in the hopes that seeding the garden well will be, if not self-sustaining, at least more sustainable. What are some good ways to do that?
Another aspect of community-building is providing opportunities for relative novices (whether new to open-source development, new to PHP, or new to migration). The proposed architecture involves myriad small, well-focused packages - an extractor here, a set of related transformers there, integrations for specific frameworks and APIs... Individual transformers, in particular, will generally be very simple. This ecosystem thus will provide ample opportunities for novices to gain experience with mentorship and also establish an online presence.
Now, all that being said, what about The Ethics of Unpaid Labor and the OSS Community (also see the recent Twitter discussion in the Drupal community)? In reaching out to underrepresented groups and to novices, we are reaching out to the people who have the least ability to work on open source for free. One way to ameliorate this effect may be to explicitly try to draw in students - whether in formal programs or teaching themselves software development - who will benefit from some free practical education and mentorship. Down the road, if this framework does start being adopted in real-world applications, we can look at ways to get sponsorships for people who maintain projects within the ecosystem.