I have originally posted these ideas in freeoffice forum but I decided to post it here as well in the hope that your dev team may decide to embrace the idea in the future as well. Both apps are great alternatives to Ms office.
Multi-grid sheets
Sheets should support more than one grid. Each grid should have its name and can support its own filtering, freeze and sorting operations . This is similar to mac numbers and rows.com . Grids could be resized to fit the table you are creating and when printing you can select which grid could be included in the print. Currently all spreadsheet apps follow the excel model, one endless grid that can be used unfortunately for only one table. The idea is that grids could complement each other in terms of information . One could grow endlessly while the other could provide analytics.
Bigger, more organized workbooks
This idea would complement the first one and it is that a workbook should be able to contain hundreds of sheets but they are not all loaded into the ram when you open the workbook, unless you designate a certain sheet to be loaded on startup. Instead it should load a sheet browser that functions like a file manager with each sheet having its own icon. You should be able to group them by letter or create groups and subgroups and direct your sheets to them . When you open a sheet , it is loaded into the ram and its tab is shown, but when all sheets are closed only the sheet browser is shown . This would allow a better ram consumption.
I understand all excel alternative want to to follow its model for compatibility. But this may limit originality.