You are somewhat right, sliding is rarely useful.
I thought long and hard about the mechanics of the sliding, there are three options here:
1) O also slides to another empty space
2) Sliding to an empty space or a space taken by your opponent
3) Sliding does not take a turn
What would you think is best?
Not taking a turn is way too strong. Guaranteed win as soon as there's 3 pieces on the board. Instead, changing board state should be something more interesting...
Maybe instead of moving a piece, you slide a whole row/column for your turn? ie top row XOX -> XXO. Now you can get surprise wins, and moving your opponent's pieces is a double-edged sword because you could slide THEM into or out of an advantageous position. Slides must consider attack and defense.
Or get weirder. Maybe something like 2048 where you slide every piece on the board in one direction but they block each other, such that they pile up on the edges compressing empty space. This one could easily be themed as tilting/flipping the table.
There's a lot of room to be creative within a seemingly simple premise.
