GoD is Gates of Discord, and is the latest expansion. It's aimed at high level players (and I _really_ mean high-level players). There is, however, a new class, Bersrkers. I've only grouped with one, and they stood near the mobs and hit them.
I might have a slightly cleric-centric view of melee classes. (Stands near the mob and needs regular healing - Tank. Stands near the mob and doesn't - not tank. Stands near the mob and needs too much healing - far too many monks.)
Lost Dungeons of Norrath is the previous expansion. That introduced "adventures" in individual dungeons (basically, a dungeon for your group). I like LDoN, to be honest. I think adventures are fun.
Prior to that, we had Legacy of Ykesha. More high-level zones, a mapping system, dye your armour.
As to what you need...um... Probably, nothing. I don't know what level your characters are, but as a minimum I'd go with Game, Kunark, Velious, Luclin. Planes of Power if you want to go anywhere. LDoN I like, but it's not _required_. GoD is purely optional.
I took a break for about a year from Luclin -> LoY. I like the game more now than I did then. YM, as always, MV.