Possible leaders of the Conservative Party instead of Benjamin Disraeli are Lord John Manners, Sir John Pakington or Spencer Walpole all of whom were cabinet ministers in Conservative governments.
The most interesting of these three is Lord John Manners (
http://en.wikipedia.org/wiki/John_Manners,_7th_Duke_of_Rutland ). An MP from 1841 to 1847 and from 1850 to 1888 when he succeeded his father as Duke of Rutland in 1888, he was a leading figure in the Young England Movement (
http://en.wikipedia.org/wiki/Young_England ), whose leader and figurehead was Benjamin Disraeli.
Young England was a political group whose ideology was a romantic and socially concerned Conservatism. It rejected Benthamite utilitarianism and laissez-force economics for an idealised neo-medievalism and Tory paternalism.
If Manners were Prime Minister his domestic and foreign policies would probably have been much the same as Disraeli's. Perhaps more "left wing".
Likely leaders of the Liberal Party instead of Gladstone are Robert Lowe; Spencer Cavendish, the Marquis of Hartington (an MP from 1857 to 1891 when he succeeded his father as Duke of Devonshire); and after the mid 1880s Joseph Chamberlain. They were all Liberal cabinet ministers.
Gladstone was the prime mover behind the Irish Home Rule bills of 1886 and 1893. Without him it is unlikely that they would have been introduced. Therefore there would have been no Liberal Unionist breakaway from the Liberal Party in 1886, though there would have been a leakage of right wing Liberals to the Conservatives from the 1880s onwards.