In order to keep simplicity and to cover most of the cases,  the Restart from Stage must keep also the environment variables from the previous build and it should use the same jenkins agent (should be optional)

why? 

environment variables - ** I believe that most of the community use environment variables to place them with data and then pass them between stages and substage and etc..

same jenkins agent - Usually we are creating artifacts like local docker images, files, logs and more on a jenkins agent, if we will use different agent, we will have to keep those artifacts on a remote hub  to make the agents stateless , this will make things very complicated and we need to keep cimplicity. so adding the option to run on the same agent will be very helpful**