Jettison the current call process altogether. Replace it with a flat system. Create a rocking good cloud-based profile sharing system where clergy looking for positions as pastor can post their profiles, and congregational call committees can search it based on their desired characteristics.
Remove the middle position (synods and/or churchwide) from the equation. Even better, move synod staff and offices back into congregations in which case more synodical staff could serve local congregations and engage in mission. Make it truly a flat system. Wide open candidacy system at all levels. Any church can interview candidates from anywhere at any time, as many as they wish. Just like the hiring processes of schools, businesses, and most of the rest of the world.
Simultaneously, get rid of the concept of "interims." Pastors considering a call elsewhere inform their congregations well in advance of their departure, perhaps even one to two years ahead of time. Congregations assemble a call committee prior to the departure of the pastor so they are ready to bring in another pastor immediately upon the departure of the present one. Since many churches experience considerable decline during overly long interims, this creates a system where strong leadership remains in place during the transition.
Again, this model is quite like the way other institutions work. Corporations do not wait two years to get a new CEO. Sports teams flag in performance when coached for too long by interim coaches.
There are of course many other reasons why the ELCA is currently not growing as a denomination. Some of it has to do with reorientation of values and theological priorities, and how people identify with those.
Even more of it has to do with the fact that we aren't having enough babies. Growing denominations and churches are growing demographically, through births, more than any other reason.
But I really do think changing our call process in the way I outline above would go a long way towards strengthening the mission of the ELCA.