In the next BB update, you should see the following changes:
* Defender no longer increases Engagement range. It was the only ability in the game that allowed an increased Engagement range, so with this change, all Engagement ranges will be the same.
* When a character breaks Engagement from a defender, they go on a list for that defender of "recently Engaged". The defender is prevented from re-Engaging that target for a constant amount of time. We will adjust this constant until it feels appropriate.
Not quite related, but connected to an issue that Sensuki highlighted: AIs were not updating their targeting preferences (of any sort) while moving, which made kiting extremely easy. In the next BB update, moving AIs will periodically re-evaluate targeting preferences within a narrow range of time. We will tune that time range as necessary and, if other common exploits become obvious, we will consider how to address them. However, a micro-oriented player with a party of high movement characters devoted to kiting presents, if not an unsolvable problem, one where investing serious programming time to it has diminishing returns.