Continuum

[Gandora]

Difficult indeed.

My first idea would be something like this:

'Draw a card, for every immortal creature on (your side of?) the field. You can't draw for as many turns as the amount of cards you drew. (except card effects).'

Immortals surpass time because they live forever. Draw is an effect advantageous to time. Filling hand and playing cards of it increases and reduces space of card slots.

[storyteller]

while this card is in play: when a creature leaves play, return its casting cost to your quanta pool.

say 3 
with a 1 upkeep

[EmeraldTiger]

You need to define what the card Continuum is going to do to space/time variables before picking which space/time variables are fitting.

But have I correctly identified the space/time variables?

[OldTrees]

You need to define what the card Continuum is going to do to space/time variables before picking which space/time variables are fitting.

But have I correctly identified the space/time variables?

No. The variables cannot be correct unless the purpose of the card (what the card Continuum is going to do to space/time variables) is defined. You chose parts before deciding what you were building.