I am not a fan of nationalisation, in fact in most cases I would be opposed to it, but bearing in mind the mess that the railways are in, I would support rail nationalisation.
The current DfT - TOC structure is not working, TPE are a good example where there is no real incentive to sort their problems out, hiding behind the 'DfT tell us what to do'. Avanti seem like another case, although I dont have personal experience of their 'service'
Coupled with (re)-nationalisation however there would need to be some major changes in order for nationalisation to deliver benefits.
Allow local managers some flexibility to tweak service provision to match and manage demand.
Ensure that all staff are onboard with the 'one organisation' message
Where there is disruption local managers make decisions based on passenger good, all the current rubbish where one TOC has to get its tickets accepted by another would be binned for example.
Work towards getting all the backroom systems into a standard format, and remove duplication both of systems and management
Re-organise and simplify fares.
Move towards a national set of T&Cs for employees, it wont be easy, but begin with all new starts being on the new terms, and then try and negotiate all the existing employees onto the new terms, but accept that it may not possible.
Try and move towards an 'arms length' organisation where subsidies are agreed at a national level, but the micro management is removed
Ensure that open access operators can continue to operate, they are one the few small success of the current system
And probably the most controversial, abandon delay repay. It encourages timetable padding, it soaks up more resources. Maybe go for a much simpler system where passengers on a delayed train are given a code or chit which entitles them to a free journey upto certain limits.
You would probably end up with management structure much like the pre 1948 system, with the country broken down into areas.