Pride
does appear in the manga albeit in one panel...
I doubt there would be an 8th Sin in the anime or manga, I don't think Bones or the Arakawa Hiromu would reinvent that particular wheel. It is clear Pride and Wrath are different in the anime though.
So hypothetically if we're talking about suggestions for a 8th Sin (out of pure fun) I would think your suggestions of the Horsemen wouldn't work, they are afterall 'processes' that can result from Sin rather than abhorrent human traits that lead to evil and wrong doing.
For example - War can result from Greed and Wrath, Famine can result from Gluttony and Sloth, Death can result from any Sin (either self inflicted or as a victim). Plague is a natural process, indifferent to humanity, so I geuss it doesn't count..?
Here's some suggestions though...
- Murder - ability to steal lives without reason
- Selfishness - inability to think for others
- Corruption - ability to corrupt minds
- Taxation - fiscal absorption
Sonhex