I think the problem with the Lib Dems is they don't know what 'Liberalism' is and don't know how to apply it practically to present day politics.
Put simply, the LibDems should be about equality without Labour's class politics. That should mean a high focus on education, public spending, economic rebalancing and devolution/constitutional change. However, they've boxed themselves, like Labour, in an awkward, unattractive corner by being staunchy internationalist and anti-nation state (they can kiss goodbye to any Northern England seats), being dogmatically anti-Brexit and not accepting to vote or at least advocating some kind of new UK-EU relationship and embracing 'woke' politics before more moderate progressivism - at least, that's what I have seen from members.
The LibDems should start by saying to the Tories and Labour that at least a third of you are probably Liberals. One Nation Tories are Liberals - mildly progressive, fisicaly responsible, patriotic but also internationalist. Whilst Labour moderates are much like the latter but perhaps more likely to advocate higher public spending. These two groups are not Corbynite or Johnsonian Red Wall purists, plus they're probably both remainers anyway.
The LibDems would then need to realise that most major socio-economic policies from 1945-2016 were Liberal in nature. The Beveridge Report for instance, Thatcherism - a Liberal rather than a Tory ideology (Thatcher even admitted she was more of a Gladstonian Liberal than a Tory), Blairism which arguably completed the neo-liberal Thatcherite project and lastly Cameron who was just a more 'small c' Conservative Blair. The Liberals have a large, ready-made ideological spectrum that if it accepts and capitalises on could drain away disenfranchised Labour/Tory moderates.
I believe they need a intellectual, passionate leader that can really aggressively sell the nation what it means to be a Liberal. Until then, they're just going to have to keep dreaming of a time when PR becomes the General Election voting system.