If it is to be a combat paladin, I would offer 1 bonus feat at 5th level.
If it is to be a non-combat related feat I would offer 1 bonus feat at 5th level, and another at 10th level. I think these two feats would need to be from a themed list for the paladin.
If your paladin makes good use of their divine spells perhaps you could make them choose from the metamagic feat list offering 1 feat at 5th, and 1 at 10th level and then add an additonal spell progression which would be:
17th level
5th level 0
18th level
5th level 1
19th level
5th level 1
20th level
5th level 1
This 5th level slot would not offer the use of any 5th level spells but could be used to metamagic lower level spells. This chart would allow a Paladin with a 20 or greater wisdom to have 1 bonus 5th level spell, which would equate to the possibility of the paladin having two 5th level metamagic spell slots, this along with 2 metamagic feats would be somewhat useful, at least as useful as the special mount is.
You could offer extra an sunder attempt.
Just some ideas off the top of my head.