I used to travel long distance with GWR, with and without DC, several times a months.
GWR used to have family carriages. I think we managed to get a seat in it once. They were a waste of time because the booking system would randomly assign bookings to the family carriage who weren't families. The same with the quiet carriage. You couldn't book NOT to be in the quiet carriage, only request a seat in it, which isn't great if you are travelling with a toddler and baby only to find you are next someone trying to work or sleep (and vice versa, I've been both). Family carriages and a better booking system that allows you to choose your seat would be good, not just state a preference, and/or allows you to state what you don't want.
The free kids activity packs were great. I would have happily paid for them.
More pram and wheelchair spaces would be great.
Buffet cars are so much better than the refreshment trolley, it's a nightmare waiting for it to come round. I also miss the Travelling Chef, so nice to be able to get real food that hasn't been reheated in a microwave. It never seemed to be advertised or announced on the train, the menus were often hidden, so I'm not surprised people didn't use it.
Please don't get rid of the Pullman dining service. Again, it only seems to be regulars who know about the amazing dining car. Make it bookable for everyone (not just first class), possibly even booking menu choices in advance to make it more cost effective. I'm sure more people would use it. Now it is either empty or so busy it's impossible to get a table, so it loses money either way.
Bookable real food, whether it's in a dining car or at your seat, would be great. We always tried to book a train with a Travelling Chef or the Pullman breakfast service when the DC were young. It broke up the journey and turned it into an event rather than just getting from A to B that helped justify the cost of the train over driving.