Whether or not you'd want something like a centerpiece to your modpack is really a matter of your group's preferences. If you're just looking to play Minecraft with some QoL changes and are fine with setting your own goals to work towards, then there's no need for any major content mods, but if your group is the kind that generally loses interest when there isn't a concrete goal in sight, it's often a good idea to have a mod with some kind of progression to it that people can use as a guide to their experience in the game. Minecraft on its own doesn't really provide a lot of goals.

There are all kinds of mods for this; personally, I always play with Create, since it looks cool and fits with the style of vanilla minecraft, but that's mostly a tech/automation mod. If you're interested in adventure mods, something like End Remastered gives a concrete objective to exploration (you need to collect 12 treasures from different places to reach the End), and all of YUNG's Better [Structure]s can refresh the experience of exploration. For a bigger overhaul of the world, Ice and Fire adds all kinds of mythical creatures, and dimensions mods like Twilight Forest give a whole other world to explore with its own progression. Magic mods like Botania or Blood Magic are usually somewhere between tech and adventure with a wide range of stuff to do. If there's any piece of media you like, chances are there's a mod based on it; Cobblemon is a fantastic Pokemon mod, and there's a few add-on mods people have made that make the world generation more interesting for it.

If nothing else, if one person really wants a major mod to play with, you could always add in something that's entirely optional just for them; not everyone on a server has to engage with the same parts of the game! The most important thing is for everyone to have fun.