I'm curious about whether the hypothetical treasure chest expansion would be designed so that it is playable with just the basic cards, either those from Dominion/Intrigue or those from the basic cards set (i.e. the same but including potion and platinum).
Maybe the answer to this is already 'no, you would be expected to already own multiple specific expansions to make full use of the treasure chest', but this seems a little counterintuitive. Unless I am mistaken, every expansion can be played by just including the basic cards, so I would assume, perhaps wrongly, that a treasure chest would be the same. If this is the case then certain cards become less likely... Potion-cost cards are not such a problem with just the basic cards set (although if you get your 'basic' cards from Dominion/Intrigue, then that still is a problem since you won't have potion); however, cards that hand out VP tokens or ruins would be a problem, since tokens and ruins require additional sets.
If cards with those additional requirements would be included in a treasure chest expansion, it seems likely that only people who already owned all of the multiple additional sets required would purchase it, else they would be buying one or more cards that they cannot play! This would seem to reduce the incentive for the set to be designed and produced in the first place. The other alternative is to supply the prerequisite ingredients with the treasure chest; including VP tokens is feasible if perhaps wasteful, perhaps even potion cards, maybe spoils, but supplying 50 ruins cards is I think much less likely since they would take so much space.