Game has a ton of potential, here's a few suggestions.
Gameplay: Tone down the RNG - Simulation. I believe this is what's causing the frustration and leading to complaints because at times your helpless when the game is influencing outcomes.
Showdown: I read the complaints as players just wanting it to play like a normal CPU game. It's clear things have been implemented/tuned to make it harder than advertised. All offline modes should play the same within their respective difficulties.
Power ups: Their already in the game with the Prestige cards and you did a good job designing it by making it possible with just stat missions. Adding more Prestige eligible cards with offline stat missions would be awesome.
Lineups: Love it or hate it the chemistry system would add so much potential to building lineups. Cards could get stat boosts when matched with same chemistries in lineup. I'll write up another post going more into depth about how chemistries could work.
Content: You guys do a good job here every year. Wouldn't mind another year of the current setup with the addition of an improved Prestige card program. My idea would be any card you earn through programs could be powered up through stat missions online/offline.
Future game additions: This is where the game could really go to the next level.
*PCI Sensitivity: Ability to adjust how quickly the PCI moves.
*Custom hitting view: Currently available offline. Gamechanger if this was available for online play. Absolutely awesome if this could be done.
*Adjusting ball brightness: Great job adding this option for the PCI. Being able to do this to the ball during hitting could really be something helpful.
Last couple suggestions: Bring back MyCareer and add MLB uniforms into DD. Being able to add Quantity to buy/sell orders.