Conceptually, I think that a table in a text document should be a spreadsheet and the content of a cell in a spreadsheet should be a text document. That way you don't need to switch back and forth between multiple applications if you want a single document but need a bit of the functionality of the other application in the suite.
That being said, I understand that you can't easily get the full thing integrated, so here is what I thing is really missing:

I would like to be able to have a table in a test document behave like a table in a spreadsheet. Not a embedded spreadsheet that needs to be opened separately to be edited, but directly in the document so it does look exactly like an ordinary table.
The functionalities I often miss in such a table are:

sort
auto-filter
at least simple operation for calculating one cell based on others

Similarly, I would like every now and then to have the possibility to use elaborate text formative some of the cells of a spreadsheet, while still having the rest still being a spreadsheet. What I mean by elaborate formatting is mostly the following:

use different format for various part of the text (title, different formats for a small number of paragraphs)
multiple links in the same cell, with one word or a few words per link
nice to have: inserting a picture in the text of the cell, with possibility to having it in line with the text

Thanks for the occasion to bring a suggestion