Once the new LHR-2 is open, LHR-1 gets demolished and rebuilt.
AFAIK that it is planned to be an extension of the new LHR-2, so the old LHR-1 and LHR-2 will eventually be replaced by one massive terminal, LHR-1/2.
After that, again
AFAIK, the long term plan is then to demolish and rebuild LHR-3, re-aligning it to make better use of the space available.
It would make sense for any potential 6th terminal to be built above or adjacant to, the existing railway spur making it the first airport station (of four), once branching off the main line.