Looking on the bright side is the possibility that the Conservatives will agree to introduce the Lib Dems manifesto proposal to raise the income tax threshold to £10,000. This would be a £17 billion tax cut - although I believe part of the cost would be recouped by the impact it would have on increasing economic growth and reducing welfare bills by making it more rewarding to get into work.
However given the level of the deficit it should be a funded tax cut through a stronger start on reducing public spending. The Lib Dem approach to fund it through higher taxes elsewhere - on pensions, Capital Gains, the Mansion Tax, etc is misguided.
Might the Lib Dems agree to it higher public spending cuts for such a purpose? The Conservative negotiating team might like to have a look at this paper for the Reform think tank by Vince Cable which details ideas for £14 billion spending cuts.