I haven’t heard much discussion of the casting of LotR.
I’d cast Merry and Pippin the other way around, for two reasons. Billy Boyd as reckless young Pippin is (to my eye) much more mature in demeanor than Dominic Monaghan as his slightly older and soberer cousin Merry. Boyd’s Glasgow accent would fit more naturally on a Brandybuck, a clan on the fringe of the Shire whose names have a Celtic flavor against the English of the Shire proper.
Saruman was noted for his seductive voice, so I’d cast Ian Richardson. Christopher Lee can be Denethor.