I'm actually thinking of building an airtable clone, on the back of a graphdb like arrango possibly, where you can actually embed other sheets in a field in a table, or link to other sheets, or just pull data from other sheets or 'bases', so that ALL data across multiple spreadsheets is accessible across all spreadsheets in one way or another.
On top of that more data types including : binaries, images, passwords/crypto-hashed fields, pdfs, markdown -- with popup markdown editor.
I'd also like to have some cool integrations ready to go like - being able to convert a spreadsheet of product listings into a complete shopify-like store. Add a product to spreadsheet, it shows up in store. (Similar concept but for Blogs). I'd also like to build in a very versatile 'event' system and api that basically makes everything zapier-ified, when data changes other things can happen.
On top of that I'd like to add some app builder that can take data 'bases' and turn them into ios/android apps, similar to Bubble.us.