@boris I completely agree that supporting autocomplete makes sense for the core, and there are use cases beyond just completing tiddler titles where this would make sense (tags for instance).
As a first step towards this the core needs to support being able to position a popup at the caret position in the editor, which is a bit involved since it isn’t something that has standards support and we have multiple editors to support. I’ve posted a proposal for how go about this and we have consensus to implement it. There is an open question I would like feedback on before I proceed on this. It is tabled while we focus on getting the next TiddlyWiki release out the door, at which time I will bump it.
Having this in place will mean both that it is a lot easier for plugin writers to implement their own autocomplete solutions with just wikitext, but also that we can then think about an autocomplete solution that might make sense for the core.
You may also find of interest this UX experiment that is somewhat autocomplete adjacent and something that I may use for my Streams plugin where there is a minimal editing interface.
Lastly, in terms of fostering better TW community engagement with all things Fission, I wonder if it might be better for posts like this which aren’t TWoF specific to be made at the TW GG.