You may note that T5 extends further west to engulf the north south road.
There are no plans to extend T5.
The building shown to its west (beyond the MSCP) is T6. Together, they would collectively be known as "Heathrow West" (also referred to in various Heathrow documents as the "Western Front Door" and "Western Campus").
Similarly T2 and, for the time being, T3 are collectively referred to as "Heathrow East", "Eastern Front Door" and "Eastern Campus".