To be honest it would just need a little creativity to spice up 5-man dungeons.
Non-linearity could mean that trash packs are not a static number on the dungeon map, but numbers / types of opponnents can change.
(E.g. You don't know how many caster mobs there will be in some groups).
Also Diablo I and II had these randomly changed "branch" points of an instance, so you would not feel exactly on rails.
Variance / fun of 5-mans will boil down to what Blizzard dev team will think of priorities - I really bet they have a priority to do better/more raids in Cataclysm, so not sure how any additional variance on making 5-mans better can come into play.