What does your feature entail? What is it for? How will it affect existing workflows or user experience?
The type of person motivated to buy the Family plan is likely to be serious enough about Kagi to also be interested in the Ultimate plan, at least for himself.
When someone is buying a Family plan, you could trigger an impulse buy that will give you additional income far into the future, each year that user continues to subscribe.
What are the exact ways that you see a user using your proposed feature? Please go into as much detail as possible, and provide examples of how other browsers/apps implement this feature, if applicable. If your feature suggestion adds on to an existing feature, how would it work into it to extend its usefulness?
I would not include it in your pricing tables. Keep them simple.
I would present it as a special offer, immediately after the initial $216 Family Plan purchase: Within 24hrs (or whatever timeframe) you can upgrade any of your six family slots to Ultimate for just $149 per slot per year.
I would guess that most Family plan purchasers would only be interested in Ultimate for themselves, knowing that the rest of their family will probably be fairly light users of Kagi. You will also, however, have instances of users upgrading all six slots. All this would be additional recurring revenue that you would otherwise not get.
You could also generate more overall revenue by discounting the upgrade of multiple slots, or maybe just having a discount for all six slots - perhaps a highly compelling price such as $499 for all six slots. I know that the compute costs per user don't leave much profit margin but, presumably, the other slots would get a lot less usage than the primary purchaser's slot. I'm guessing that most Family accounts don't even activate all six slots.
You could also extend the same offer to your 900 existing Family subscribers on Black Friday. It might even be an effective way to get existing individual subscribers to upgrade to Family.