It's not difficult to find improvements to LLMs still.

Large issues: tokenizers exist, reasoning models are still next-token-prediction instead of having "internal thoughts", RL post-training destroys model calibration

Small issues: they're all trained to write Python instead of a good language, most of the benchmarks are bad, pretraining doesn't use document metadata (ie they have to learn from each document without being told the URL or that they're written by different people)