Horizon Lunar Colony is one of Overwatch's most-hated maps. For starters, it's an Assault (two capture point) map, the gametype that "everyone hates" due to the incredibly close defenders' spawn location. But on top of that, the overall map design has a fair number of problems.
It looks like Blizzard has taken notice, as a map rework is currently underway, dramatically changing the potential attack and defense avenues surrounding each point. Several flanking routes have been changed—some opened, some closed—and there's more available high-ground access sports. Here's a video flying through the updated map on the current Overwatch PTR:
Interestingly, these changes weren't actually intended to hit the PTR just yet.
"Well that wasn’t supposed to happen but there you go," said Overawtch principal level designer David Adams on the forums. "We have some changes that we are still working on and testing for the map so what you are seeing here may not be final. Also as an FYI it won’t be released with the next patch and no eta for when it will be released as it is still a rework in progress."
The current Overwatch PTR has a big pile of hero balance changes, including the long-awaited Hanzo rework as well as small nerfs for Tracer, Genji, and Junkrat.