I would say A.I. and Franchise. I expect big improvements to franchise because they never leave one area or other areas neglected or at least havent in the last 2 years I've played the game. They are really good about improving all areas of the game, I don't know why anyone would try to make a post about asthetics vs gameplay.
But for the gameplay side there are lots of things that can improve. You pointed out a big one in the doubles down the line physics which makes guarding the lines unnecessary ever and removes a strategic part of the game that I'd like to have there. I love the little strategy of baseball and making the right call at the right time to win the game. The more the better.
When it comes to A.I. I wish they would use their bullpen better/smarter. How many times have you seen them bring in a LHP to face a RHB? They are oblivious to batter pitcher matchups and ratings. They spoke about manager AI in an earlier stream but didnt elaborate. I think that may mean something like how frequent certain managers do hit n runs or steal, which would be nice to have. I am hoping this alleviates the issue last year where the CPU would double steal quite often and often in dumb situations like 2 outs. Even with slow runners. They touched on stealing this year so I expect it to be improved.
Something that is a tall order and has never been done in a baseball game that I would like to see is a way to differentiate batter swing speeds. In real life certain batters have a hard time catching up to fastballs so pitchers throw more of them when facing them. But in the game the AI are all equally good/bad versus curves sliders fastballs. I prefer online because playing a human lets you feel that. Seeing the guy late on his swings to your fastball makes you pump more at him. I want that same feeling against the AI. Imagine in franchise as a player gets older his bat speed slows down and he begins having trouble versus fastballs when while he was younger he made his living off of them. I don't even know how it would be done but I think it's a big part that no one has tried emulating yet. Other sports games have moved from strictly ratings and brought more dynamic gameplay by using traits to further differentiate players. That's harder to do in baseball but I think this idea would help in that regard in a big way.
For franchise mode in the current state (MLB 15) I would give it a B+ but it certainly can be major improved. I think The Show has the best simulated stats of any sports game but last year there were issues of complete games/shutouts being x2 the amount as you'd see in real life. They mentioned they fixed progression which was a bit OP for pitchers last year, I hope they got the CGs under control as well. To me sim stats are so important, what is the point of playing all those games if you can't really compare yourself to the rest of the league?
There are lots of little things in franchise mode that could be added such as Non-roster invites to spring training, contract things like control over how the money is payed out year to year. Lots of little rules in the CBA that are technical but cool to have in. Schedules that change every year in franchise mode instead of repeating. Teams using their pitching rotation more accurately in playoffs... like a 3 man rotation. Bringing a SP in as a RP at the end of a series. Agents for players would be a cool feature. Where some agents make their guy hit the free agency market 9/10 times, and others are more team friendly. Franchise sliders would be huge. Really glad they are adding a trade slider but there can always be more. If you have played NBA2k16's MyLeague you know what I mean. That sets the bar in that area. Injury length/severity, trade difficulty, simulation difficulty, draft class quality, player progression and regression rates, training effects, and morale are ALL user controlled.