As far as I know there would be three basic options, or combination of options, as far as making standing possible at what are nominally all-seater stadia: bolt-on seats, fold-away seats or German-style rail seats. The third seems the easiest and neatest for Anfield, but you'd probably also need to incorporate surge-resistant barriers. But I get frustrated when the practicalities are discussed because they all seem to focus on how to incorporate safe standing, whereas surely the more pertinent practical issue is how to also incorporate safe and quick evacuation. That's certainly the issue that makes me feel a cold shiver down the spine whenever I think about the topic. I've seen loads on how to deal with the seats, but hardly anything on how to deal with evacuation.
The Taylor Report noted that there's nothing intrinsically dangerous about standing in a stadium, but it also noted that it's never possible to completely rule out standing, and swaying, leading to problems that could be dangerous. Maybe it's a generational thing, or just my irrational and over-emotional perspective, but, for me, even with stadia that are better-built, sturdier and more regularly inspected than ever before, I'd need absolutely ineluctable evidence that they were also equipped to ensure that there could be no possibility whatsoever of constriction and obstruction at the front (or, for that matter, any other part) of the terrace. I guess that's possible, but I've not personally encountered anything that is currently in use.