The Lib Dem's main focus for at least the last 30 years has been electoral reform, preferably to a proportional system like STV which doesn't punish third-parties. If nothing else, a Lib Dem government managing to pass that would have a massive impact on the future of UK politics, and break the two party system that first-past-the-post inevitably causes (whether Whig/Tory, Liberal/Conservative, or recently Labour/Conservative).
They took a huge gamble entering the ConDem coalition: getting some concessions from the Conservatives (e.g. same-sex marriage), but overall achieving hardly any of their manifesto promises (which, indeed, included scrapping tuition fees; first mentioned on page 33 https://www.markpack.org.uk/files/2015/01/Liberal-Democrat-m... ).
It seems they gave away way too much in order to get a meagre chance at voting reform. The resulting referendum was watered-down to Alternative Vote (rather than STV), and was not only attacked by Conservatives, but also Labour (who had ignored their own manifesto pledge in 1997 to follow the Jenkins Commission recommendations, which were to use a variant of Alternative Vote!)
There are some grass-roots efforts to support electoral reform in Labour; I doubt they'd do it themselves (like they didn't in 97), but maybe they'd be more amenable to accepting it in return for Lib Dem support?
My main worry about a Lib Dem government would be how much influence their 'free market libertarian' wing would have; where 'individual liberty' becomes a synonym for 'deregulation of the private sector'. There's definitely been more of a such rightward-lean since Clegg (compared to e.g. Charles Kennedy).