Question in the topic.

i know it’s labeled as an expansion but I was wondering if it could be played with just the content inside the box.

Not quite. Apart from tokens (which are easily replaceable), there is a set of cards from the Core that is meant to go in every villain deck to add basic abilities (e.g. every character has a Nemesis, that set is the only way for them to enter play).

I think the game would be playable out of the box, but not quite right. Also, there wouldn't be much variety -the hero cards in the box are mostly for the Hawkeye/Spider-Woman decks, so there wouldn't be any real deck building.

The only scenario so far that do not need any modular set from the core box is the Wrecking Crew. You can just buy a hero pack and a pack of Wrecking Crew to start playing (given you have some token and condition card replacement).

RORS requires the core set, source: the box.

Some will say that you can kind of play it without the core set, but it means not having the standard encounter set (which every scenario explicitly requires and includes the only card that can bring in each hero's nemesis), Legions of Hydra modular set (which Crossbones requires), and making your own tokens and status cards (which is actually not that hard). To me it seems like playing Marvel Champions scenarios wrong but with extra steps.

The good news is that the core set is amazing value with 5 heroes, 3 scenarios, 5 modular sets, tons of great staple player cards, and all the components you will ever need to play. Once you own the core set you can play any expansion with no issues. If cost is an issue, there are a bunch of used copies of core set available on BGG and Kijiji these days, depending on where you live of course.