I miss the feature in readline (Bash/Zsh) where Ctrl+x, Ctrl+v switches to Vi command mode temporarily. In that workflow, entering Insert mode switches back to Emacs mode. PSReadLine has a command ViCommandMode, but binding it directly in Emacs mode will report errors on every key input. The solution requires handling the mode change event to toggle the global EditMode between Emacs and Vi.
There are two TAB’s in Emacs, (kbd "TAB") (\t, [9]) and (kbd "<tab>") ([tab]). If modes like yasnippet and auto-complete want to bind on TAB, their trigger key must be the same with the original Tab command. Since Emacs binds indent-for-tab-command on (kbd "TAB"), so it’s better to use it as the trigger key. Yasnippet binds to it by default, It is also easy to setup auto-complete to trigger using Tab.
Yasnippet tries functions in yas/prompt-functions when it needs user to select one choice, such as selecting snippets with the same trigger key, such as helper method yas/choose-value. popup is a visual popup interface library extracted from auto-complete by its author. It has better look and feel than all the built-in yas/prompt-functions. Also it is easy to customize, and its isearch mode is very efficient, the items are filtered on-the-fly when typing.
Not fully tested, let me know if it mess up your buffer. ※ my-ruby-mode.el (font-lock-add-keywords 'ruby-mode '(("\\(\\b\\sw[_a-zA-Z0-9]*:\\)\\(?:\\s-\\|$\\)" (1 font-lock-constant-face))))