I think a lot of people feel the franchising system as it stands is bonkers and doesn't really encourage long-term private investment in the railways. Typically, franchises are only let for seven years or so, but the more basic problem is that most of them are not 'vertically integrated'. Most franchisees don't own the actual trains, nor do they have any control over the tracks and other infrastructure apart from station buildings, so the scope for change and innovation is quite limited. Major train orders and infrastructure projects are ordered, paid for and specified (often badly) by the government.
There are a few exceptions which are usually held up (at least in the railway press) as examples of how things could be done better. The one that's usually quoted is Chiltern Railways, which runs all services out of London Marylebone and has a much more comprehensive remit than most franchises. They have invested in new trains and track building so as to greatly improve services to Oxford for instance.
You could argue that a single nationalised institution such as BR represents the ultimate 'vertically integrated' system, but that brings its own problems. For one thing, the only major part of the railways that is currently nationalised is Network Rail, an inefficient and risk-averse organisation that has completely ballsed up all the recent major electrification programmes. Minor infrastructure projects such as building new platforms or bridges have to be done through NR and they work out incredibly expensive. There would be no benefit in making the rest of the railways more like NR, if that's what nationalisation means.
Another potential issue with renationalisation is that we are now an entire generation on from BR days. There are no longer many people around with the experience of running an integrated national rail network, which obviously is a hugely complex business. There's also the fact that, bizarrely, some aspects of UK rail privatisation have been taken as a model for EU legislation. It would actually be illegal to renationalise the UK railways along BR lines while we remain in the EU.
So although it might well have been a privatisation too far in the first place, and the franchising model is broken, the question of how you replace or fix it is not so easily answered.