Already I have seen missions which are just made to be places. I've seen fleets make their own Starbase maps or someone just make a deck of their ship with the favorite things.
I see no issue using newwly created assets there or even cstore content (if applicable) even if you have not purchased it. For one thing, to be fully immersed you will go about buying/earning it yourself.
I don't know if there are plans ofc, but I would assume we will only be charged for mission slots and not access.
The OP's question is legittimate. Think about it? What would happen if STO went F2P?
In any case, we are already PAYING to produce free content for the Devs.
Or think about this hypothetical breakdown:
$14.99/ mo gives you access to the game only.
$18.75 gives you access to the TOS interiror and shuttle (due to limited time only, costumes does not apply)
$25 gets you the referral award ship
$X amount gives you access to exclusive Foundry controls
Considering the Foundry is free content, I'd say they'd make mission creation free (but have a lower cap of missions on bronze accounts) then they'd have a finite number of Foundry missions per day or per month that you could play on a free account.
No, the Foundry is what makes STO stand out from all other MMORPGS (except that it's Star trek, of course).
The only thing I can see for the C-Store are additional mission slots. Maybe even a premium-mission slot that can have 20 maps or whatever. But that's it.
I am certain that Cryptic will not put Foundry-assets on the C-Store.