There's already a thread about this here: https://forums.ubports.com/topic/216/security-updates-for-ubuntu-phone-to-end-in-june/2
Basically, we will use the OpenStore, an alternative application store for Ubuntu Touch that has already been around for a while. It can already be installed and the most important apps have already been moved over there.
It will be improved in future releases to offer easier navigation and better look and feel, but all the basics are already there. The OpenStore will be preinstalled in our future releases.
Please continue this discussion in the other thread.