This could allow players the chance to improve their own knowledge on selected (unlocked) lists without having to wait for the list to be randomly selected in-game for matches.

In this game mode opponents would not need to be selected for the player, as it acts as a sort of ‘sandbox mode’, whereby a list (and maybe even letter?) can be selected by the player at no in-game or real life currency cost.

I understand how this feature may not align with the Scattergories vision, which is why I might suggest something like a 1 hour cooldown period on these practice games. This ensures players continue versing online opponents (as opposed to taking this proposed cost-free feature for granted), therefore likely having the effect of leaving players wanting, and receiving, more playtime. Win-win.

And then maybe in a future update, a new in-game prize could be introduced that acts as a key to unlock extra free-practice rounds, which would essentially negate the cool-down period of games for those lucky enough to win this prize.