- The third person plural set (they, their, them) was borrowed from Scandinavian (it's pretty uncommon for languages to borrow 'core' vocabulary such as pronouns).
- Old English (Anglo-Saxon) also had dual forms of the pronoun. Those were lost by Chaucer's time though. Not sure if they are reflexes of early PIE dual forms or a more recent development from Proto-Germanic.
[Sara Malton] [1] has some interesting ideas about how the you/thee distinction developed in the semantic dimensions of power as well as familiarity in Early Modern English.
There are many new 2p. plural forms developing in English that vary regionally and socially, e.g y'all, you guys, yous, you'uns (-> yinz), all y'all.
[1]: http://homes.chass.utoronto.ca/~cpercy/courses/6361Malton.ht...