You wrote "better completion engine" as a point on your list.
It's not really clear what the problems with the current ones.
That's why I'll write the problems I have with the most popular one these days nvim-cmp:

• bad configurability: The way this works now is that there are basically just three fields (kind, menu, abbr) which I guess originated from just lsp completions
  This has many downsides: perhaps you want to display other things which then will have wrong names, you can just display three different things, you can just have three different highlights so it's e.g. not possible to have half or rounded blocks around icons
  A pull request allowing to have more custom fields wasn't accepted hrsh7th/nvim-cmp#1238

A much better way would be to use a completely new format which I used in a completion-engine project I started a while back:
You basically just return a table with text, highlight tuples
This allows things to be completely configurable and also can directly be used in buf_set_extmark
see here https://github.com/max397574/neocomplete.nvim/blob/main/lua/neocomplete/config.lua#L57

• this is just a personal thing but I rly don't like the object orientation of sources and in general the way sources are written could be much better