The Voyager rolling stock is great, very reliable and a pleasant journey. They are due a refresh inside though! The walls could be painted white and the purple ceiling in the vestibules painted over. I'd also change the oak effect on the tables and light chamber strip, as it is very dated now. Other than that, they are comfortable, well equipped, a smooth ride and don't have the awful LED lighting that is now popular. I'd remove some of the table seats to squeeze in more airline style seating, having a maximum of two tables per coach in Standard. Four coaches is enough if everyone keeps their belongings off the adjacent seat to allow others to sit beside them.
I'd have more services heading north from Plymouth/Penzance until later in the evening and I'd also re-introduce the South coast - North East and Manchester to Bristol services.
The catering could be improved. First Class needs a main meal option, even if it's just a large salad, they had good feed back about their chilli when served it.
Hopefully, they will acquire Avanti's Voyagers and use these to retire the HSTs. The Turbostars could do with having First Class extended to more of the coach, guaranteeing that a First Class ticket will be honoured. Again, catering could be improved to salad meals and perhaps a fruit salad/porridge in the morning.
What they need to capitalise on is the fact that most of their trains run right in to two shopping centres, Grand Central & Bullring. It would be good to see this being advertised more on social media. If I recall correctly, there is covered access direct from the station!