No carry on - Literally nothing? Either a total badass or a terrorist.
Small bag or backpack (messenger, purse, etc) - Pro traveler. Knows what they need.
Small roller bag or duffel plus personal item - Quick trip or maybe on a tight budget. Either way, handles their shit.
Large roller bag and personal item - Probably also checked two bags. High probability they’ll put both in the first available overhead bin even though they’re at the back of the plane. Lines up as soon as boarding starts even though they’re in the 2nd to last group. So... The majority of travelers.
Large roller bag, large personal item, jacket, blanket, hot coffee or other spilly beverage - Burden to themselves and everyone around them. Has literally never placed an item in the overhead bin themselves, they just stand in the aisle and struggle pointlessly until someone helps. Bag has never successfully fit in an overhead bin but still gets outraged when asked to gate check it. (AFTER failing to get it in the bin) Asks to switch seats with you because they just can’t sit in a middle seat. Might have a small dog or child as carry on. Lacks all common courtesy and self awareness.