I personally think it is better to go with adding cards that can provide mobility rather than reworking existing creature abilities to allow for movement. At least at the offset.
The exception to that, would be if this were done in the context of adding new effects that modify existing mechanics.
Some examples,
-adding a card that allows creatures to move to new slots when burrowing / unburrowing
-something that forces creatures to move into the slot opposed slot of any enemy creature that they target.
-A spell or buff that allows a creature to evade targeting by moving to an adjacent unuoccupied slot, which would mean an empty slot to its left or right must be available for the evasion to trigger.
--Could make both an amusing alternative and nerf for SoFr
As for the 'central card' idea, I mainly posted the two cards as examples. My main point is that it would be good to make sure that every element has some form of access to movement / positioning mechanics. A splashable
card is a great way to start, but there should definitely be plenty of other options as well.
This is actually a great way to help further define elemental themes, in terms of how they go about affecting the field.
E.g. gravity could be focused on pushing / pulling, air could use movement for dodging, entropy could have fun with shuffling / randomizing creature positions, etc.