There are things that could be added to Vanilla Minecraft. There are also things that are balanced and don't affect the gameplay of Minecraft. Then there are well-balanced things that you just have to have in your game. Originally, this was a modpack intended to add things that had a vanilla feel to them, but I eventually gave up on that concept as I found more and more mods that just had to be added. I still try to maintain the basics, like diamonds being one of the best materials there is, and players collecting wood in the beginning. Other than that, I've added a ton of metals (few of which are better than iron; even fewer are as good as diamond), and a whole new way to smelt ores and craft tools. Plus several new dimensions and an immense assortment of hostile mobs. This pack is not overpowered. In fact, its significantly more challenging, as you have to avoid running into shoggoths, falling out of the world (sorry, no more bedrock layer), and more. Unless, of course, you feel confident enough to face these dangers...
Anyway, this is a highly adventure-oriented modpack, so I'm mostly looking for mods that add dimensions and biomes. However, I am very willing to look into all mod suggestions. So suggest away, and don't forget the #1 rule of gaming: have fun!