As you said, the social and dating aspect (I'm going for Leaf/Hilda/Serena too) is most certainly important in this kind of event. I merely intend to provide a template for the devs to work into the overarching story. It would be worth discussing how the dating aspect would fit into this. But first, some context.
I'd imagine that the player will spend time designing, planning, and constructing their event. If they are part of the student council, the player may be attending meetings to plan the festival as a whole. Breaks would consist of checking out the other events with a companion or two. You would still have access to all the places you go to during regular breaks (as seen in one of the recent update videos), though the backgrounds would be edited to include students setting up their event. Going to the city could allow you to discuss the festival with outsiders, learning what theme most people will be interested in. Once again, this is merely a template. It does, however, provide some much needed time to spend with friends or potential lovers. As for spending time with them within the clubs, I'll be using the outdoors club you mentioned.
Leaf and you are tasked with some duties which involve exploring a nearby forest. You may be setting up a haunted forest theme, where friends and couples brave a maze of trees at night to reach the end. Along the way, props, trainers, and their Pokemon are made to scare them (I'd love to see Leaf's Jigglypuff try to take on that role). Anyways, you two meet up past curfew (with knowledge of the consequences) to look for the scariest section of the forest at night. Despite your possible complaints, Leaf drags you along like she always does. As you sneak past lazy security guards, Leaf teases you for being so nervous, as the guards expect nobody to be lurking around; the rumors mentioned by Leaf in the game have proven to be effective enough. Once you get deep enough into the forest, you may decide to give her a taste of her own medicine. Leaf doesn't notice you sneaking away, and she is left alone in the dark. You "test out the site", or in other words, scare her to death.
Depending on how you execute this prank, Leaf's reaction could be quite varied. She could think that you're an asshole, or she could be impressed by seeing this different side of you that can compete with her flirtatious and aggressive attitude. There are a plethora of ways that the whole event could go, based on your decisions of course. You may get lost in the forest mid prank, or Leaf picks up on your attempt at a prank, scaring you instead. She may have been planning to scare you all along, as you might find if you choose not to pull the prank. On a more extreme note, there might be some nefarious activities going on in the forest, which Leaf and you could accidentally stumble into. I would personally want to see what happens between Leaf and the player in a dark forest. They're alone with only each other's company. Leaf may expose a side of herself that she hides from the public eye. She may reveal a part of her past, which has been hinted at in her character summary. Things may even get a bit steamy... I mean, anything to further develop their chemistry.
Two more things to add. First, the battle club (that most players will choose to join) won't be in the way of your second club. Either that, or it is part of a main story arc that almost everyone will go through. Other than that, it is open for outsiders to take part in a tournament, for possible students to be sighted for their potential, and for you to have a battle with that girl you met in the city at the beginning of the game. This tournament will be a great opportunity to level up your Pokemon that you have since neglected in a week-long preparation for the festivities.
Second thing to add, you won't be stuck at your event during the entire festival. You work in shifts, leaving you with a choice of people to spend time with. Some event options may be turned down by your friend. Serena may enthusiastically accept your offer to go the Ball (or other dance when you're not in Tierno's club), but Cheren could sigh and walk away, taking it as a dumb joke. As for the places that your friend or partner will accept going to, there will be no best option. Different stats could be raised in certain places, but that's it. What really matters is what situation you want to experience with your partner. How would Leaf act in a fancy ballroom? How would Hilda manage in the haunted forest maze? If you remember Leaf's hesitance to go to the pool, it makes me wonder what event the swimming club is holding, and whether she will finally put on a swimsuit. One thing is for sure, though: Brock will be there. Watching.
In any case, this is a huge section of the plot. A week of preparation, 3 days to a week of the festival, and the festival is held once a year. While this is a massive undertaking, it will surely be worth it. A lot of scenes will need to be made, from the art and music departments to the scripting and writing departments. Much of this material can be used for the next two festivals, where you simply choose routes you didn't get to see yet, but with much higher stats and responsibility.
So? How is this for an idea? I'd like to hear your thoughts.
I'd like an order of Personal Space, please.