Looking a lot like diablo/poe seasons. Since we don't have multiple characters I wonder if all seasonal stuff is transferred to the main at season end.
1. If you take the Personality Vacuum perk, you will still be able to obtain Kappa.
2. Connection to the Arena. It will exist, but with logic similar to PvE. You can gain experience, skill experience, and mastery there, and transfer items from the Arena into Seasons (but not back from Seasons into the Arena). Some Ref quests that send you into the Arena will be unavailable in Seasons.
3. Modifiers apply only to PMC.
4. Prestige will not be obtainable on a Seasonal character.
5. Allergy works like in real life: you will have to figure out what you are allergic to. Allergy applies only to meds, stimulants, drinks, and food, and does not affect medkits or CMS. When the Allergy triggers, you first receive a hand tremor effect, followed by poisoning that deals damage to your health.