Understanding the "vibrancy needs" of people living in the cities in question (be it Detroit or elsewhere) is a crucial start to knowing which of those amazing concepts already posted here can work best. My inspiration relates to the process of doing so. One interesting way to do this is by applying a completely participatory approach to sense those signals from the people and then design a systemic solution - out of many existing solutions - together.
Why not creating an organisation which does just that? And make it a movement. Firstly it understands inhabitants wishes around vibrancy, revitalisation and success, through a variety of ideas and participatory techniques. Then it supports those wishes and make use of people's intrinsic motivation to make it happen, bringing together a variety of solutions to address those needs. There are so many great solutions posted here already. In many cases it will only be about bringing the right people together.
As an example, MAIS UM is a small community development project operating in Coroado a 2,000 people community in Rio de Janeiro, Brazil. This project (which is now rather a movement) strategically makes use of a variety of events in Coroado primarily to get to know the community inhabitants, their dreams and their hopes, to understand how to best help them deal with their most pressing challenges (Check out their strategy: http://coroadoproject.blogspot.com/p/coroado-project.html).
The project has created a movement around the community and grabbed volunteers from inside and outside the area who became committed to the cause. Events like parties for children, flower arrangement lessons for the elderly and a cleaning day at the community centre all come with a bunch of volunteers who are "instructed" to actively seek for insights from the community (which is often sceptical of change and pessimist). Those insights help the volunteers to design projects which address exactly what they need the most - together.
After understanding well their needs, the volunteers and coordinators of MAIS UM make use of their personal networks to connect other NGOs, companies, religious organisations and the government to come together and address the challenges of the community with the community participation.
As an example, in thinking on how to address the literacy challenge among elderly in the area, a volunteer indicated a literacy teacher and MAIS UM found a project from the local Rotary club which provides stationery materials for students and a stipend for the teacher. With a bit of coordination from MAIS UM, the teacher met the Rotary club, elaborated a course plan and started the free lessons for 30 elderly in the community centre (photos above).