Anderson’s straight over the top motion is going to make adding pitches difficult because he won’t be able to get much horizontal movement. That means no slider or cutter, and no sinker with any sort of real arm side run.
His current mix of rising FA, downer CU and downer CH are pretty much the optimal mix for him given that arm slot. I don’t think a split would help because that’s effectively a CH.
No idea if he can change the arm slot, but I guess it wouldn’t be worth it. His extension on the FA and deception on the CH makes them an effective combo, and that’s already a very good SP with a usable CU.
My recommendation would be to continue refining his command/execution of those pitches, and continue being a good SP. Like most pitchers, he will likely lose enough stuff in his 30s that he will no longer be effective, but just ride him for now and enjoy the results.
As striker said, do you think Ian could perhaps add a splitter or forkball?