Hello,
as is usually the case here in the forum, there are two entrenched factions for basement yes/no. That can be discussed forever.
To help you with the existing plan now, here are a few ideas (and assumptions—did I miss the answers to the questionnaire?) from me:
Parties, especially children’s birthday parties or anything that requires frequent trips to the kitchen, are best on the ground floor; children and smokers can also go out into the garden if needed. Put beer benches next to/instead of the dining table and push the couch aside a bit for a larger party (which will probably not happen more than three or four times a year anyway). In a pinch, you can set up a heater in the garage if it really doesn’t work in the living room.
I assume that so far no large exercise equipment is present, otherwise you probably would have already considered it in the planning? If a stationary bike should come later, it can possibly also be placed between the dining and living areas.
Flea market boxes can, just like holiday souvenirs, go into the utility room or under the roof. However, I always wonder with stuff like that whether one should just throw it away right away or not even buy it? Otherwise, my (few) holiday souvenirs do not end up in boxes but rather decoratively on shelves. ;-)
If you could actually bring yourself to close off the open space and make a storage room out of it, you would have additional space for flea market items and seasonal clothing. But I would also redesign the bedroom (possibly forgoing the partition wall between the dressing area and the bed?) and instead accommodate more wardrobe space there to minimize seasonal wardrobe swaps.
If you want to give up the open space but not the lighting for the hallway, a stationary bike could also stand there—next to shelves/cabinets on the wall facing the bedroom. It still looks more open than with a storage room but offers at least one or two more meters of cabinet space.
I have no experience with saunas, but I am wondering whether something like that could also be retrofitted in a children’s room—when the children have moved out. Then at least there would still be space for lounge chairs.
For hobbies, nothing comes to mind at all, which is mainly because I don’t have any space-intensive hobbies. I can’t really imagine it for you either, as you probably would have already considered it in the planning otherwise...? Otherwise, the children’s rooms can also be used for that once the little ones have flown the nest (okay, that may be far in the future, but I assume space-intensive hobbies are still far off).
Regarding direct garage-house access: which priorities are more important to you? A 5-meter shorter walk with groceries once or twice a week, or at least one meter of aisle space for more storage? You can only get an additional passage by giving up wall—and thus shelf—space. I would probably prefer to walk around outside...
Otherwise, a tip: walk through your current apartment with open eyes or even a tape measure to see how many cabinets/shelves etc. you currently need, where you actually lack space, and how/where you concretely want to store your belongings in the new building. For example, in my living room there are at least three meters of shelves that would be completely distributed to other rooms in the new building: folders to the office and glasses + “decorative” alcohol bottles to the kitchen.
Finally: just take a deep breath and give up the idea of having to consider EVERY eventuality from the sauna to the model train collection to your own fitness room. Most people can’t have everything, but for many things a solution can be found when the time comes. :)