Currently, the plan is that the UI for upgrades to Drupal 8 from Drupal 6/7 will be initially developed in contrib, to be pulled into core when "ready" (a decision will be made before RC whether it will be made available in 8.0.0). This UI is under development as migrate_upgrade, and at Drupalcon Austin Bojhan and webchick sat down with me to review the current state of that effort. From that session came a punchlist of suggested changes to the UI as it is now. I'll be adding those issues in the sandbox as children of this meta issue - the meta is here in core for visibility, since this work will eventually end up in core. Note that many of these will be Novice issues, for those looking to get started contributing.
For historical background - previous discussion on the UI leading to its current implementation - see.