The things that are important to me, may not apply to you. I have 2 kids, 8 and 11 but have traveled for up to a month with my niece and her 2 kids that are the same age.(And doing it again this summer)
1. Walk around queen bed.
2. Couch for plenty of seating and sleeping.
3. Enough counter top for food prep and dishes.
4. Bathroom with enough room to move in.(Don't have enough room around the toilet now)
5. Clothes storage for 8 people.
6. Separate room for sleeping kids or visitors.
7. Light enough to be easily towed with a 3/4 ton Suburban and while carrying up to 8 people.
8. LED lighting to keep the power usage down when boondocking. Kids tend to leave lights on occasionally.
My TT accommodates a large family nicely. But when I retire, an open rear living area will be a requirement.