I play Sparky to get a feel for my decks, but the vs-AI mode could be a fun, more challenging aspect to Arena. There is really a lot of untapped potential here. Here are some thoughts.

1. Give Sparky more recent cards. Maybe from the recent beginner decks. The cards feel underpowered and dated. It just isn’t challenging playing with the most recent cards against Sparky’s older sets. I have noticed that the programming only allows it to play cards that have direct interactions (for example, Pacifism on one of my creatures). It is not programmed to play cards with activated or triggered abilities. This limits its card pool and ability for more realistic, clever play. Sparky will never be able to play a Planeswalker in its current state, for example.
2. Give Sparky difficulty settings. For example, give Sparky really good tournament decks from the most recent event and that would be the “hard” setting. Such decks would also be fun to play against with our own decks. This would also make the AI evergreen.
3. Make Sparky smarter. Sparky had me beat, when it had two attacking cards vs 1 defender; it dropped a +2/+2 white card on the attacker who was blocked, probably because it is programmed to do so. It did successfully take out my lone defender this way. However, the problem is that it should have used the buff card on the unblocked attacker to take me to zero and win the game, as any good human opponent would have likely done.
4. Change Sparky to give the AIs differently personalities and playstyles. Chess.com is a good example of how this can be done. Each AI player has different emotes, which would add some variety and charm. Maybe make them Planeswalkers.
5. Sparky is particularly bad at endgame scenarios. For example, if I am leading in health, it will attack me with all of its creatures, doing damage certainly, but doing so leaves it completely open to a game-winning attack on my turn. It should learn to block in such situations.
6. When Sparky wins, it is noticeably because I got mana screwed, or drew all forests in a green-white deck. I guess this can happen to Sparky as well?