FE8 has a few to choose from, all of which having different features. Sacred Trilogy and Sacred War both add skills and a ton of new weapons and classes, while still keeping the story (mostly) intact. Sacred War specifically has overhauled the postgame in a very substantial way, if that interests you. There’s also the aforementioned Princess’s Lament, which adds a lot of characters, story rewrites and extra dialogue. I’ve been playing through TPL and it’s lining up to be my personal favorite of the three, however each one has it’s merits and I’d recommend reading through the features for all of them.
FE7, I’m not sure about.