Pantyffynon - Llandovery
Doleau - Llandrindod Wells
Both of those would allow a 2 hourly frequency between Swansea/Abertawe and Llandovery, plus a 2 hourly Cardiff Central - Llandeilo or Llandovery to operate, and also a 2 hourly Shrewsbury - Llandrindod Wells to operate. The central section between Llandrindod Wells and Llandovery could then have a frequency of every 4 hours through extending the Swansea - Llandovery and Shrewsbury - Llandrindod Wells to meet up (I worked out a possible clock face pattern a while ago - "West Glamorgan and Dyfed" was the first part of the title of the thread I created)
Also in Wales, I would get Chester - Wrexham General and the Ebbw Vale doubled properly.
Furthermore, I would also double Machynlleth - Dyfi Junction and Machynlleth - Tallerdig Summit (or Caersws, as it looked it was double at one time) so as to provide robustness for the 60 minute frequency between Aberystwyth and Shrewsbury, and also a few extra minutes in the bag for the reopening of Bow Street Interchange between Borth and Aberystwyth).