Should need no explanation - a widget that behaves similar to (or ideally, exactly like) the stock touch bar predictive text interface, including automatically appearing when the user is typing, support for predictive emoji, contextual suggestions, and autofill suggestions. This is the one major thing missing from BetterTouchTool that I truly miss from the stock touch bar.
Unfortunately it is unlikely this can be added.
BTT doesn't have the same access to entered text as the system touch bar has, and also creating an algorithm to predict the next words would be it's own project.
Maybe if I ever find a way to integrate the system predictions, but currently I don't think this is possible.