Is your feature request about something that is currently impossible or hard to do? Please describe the problem.

Vim has <Tab> completion for cmdline, but I still want autocompletion (like it will show the popupmenu on every key I type). It is possible to implement that using InsertCharPre event with feedkeys(), but the problem is that Vim will automatically select an option from the suggestions and automatically insert it to cmdline, and there doesn't seem to be a way to prevent that.

Describe the solution you'd like
Options noinsert,noselect for 'wildoptions' that work like 'completeopt'.

Describe alternatives you've considered
Another way is to implementing it from scratch using popup API, in fact, that is how plugins implement it, but that just seems like reinventing the wheel