Currently the API is very much limited to what is possible in the frontend in my opinion. It would be great if you could add events like:
- A cell/range was selected
- A cell was cut/pasted/copied
- The context menu was initialized
- A row/column was added
- A row was added/deleted/hidden
- A cell's content was modified
...
...

Set default Cell Types like Date(Time)Picker, Checkbox, 
SimpleCombo, Autocomplete or own Custom Editor (see other Feedback suggestion)

I know that Kendo Spreadsheet works like most popular Spreadsheets (Excel and Google Spreadsheet) where, if you copy or drag and paste values from cells, you will copy also the styles, validations, etc from that cell to the new one (unless you select "paste values only" in a context menu or ctrl + shift + v). 

It would be awesome if we can set how this works by configuring a property in the Spreadsheet configuration, something like pasteValuesOnly: false/true or even more complex with an object where you can choose if paste values, validations, formulas, styles or whatever. Also you could attach this functionalities to a context menu (as Google Spreadsheet does) and be able to paste everything, values only, format only, formulas or data validation.

It would be great if you could put images to cells using html. E.g.: <img src="..."> would result in an image in the cell.

Ability for spreadsheet row height to be auto set from the row contents. This is to support wrapped text.

For KendoUI Spreadsheet, it should have the ability to add images or logo into the spreadsheet. This feature is already available in Handsontable component.

Use case:
My app doesn't know how many rows of data the excel spreadsheet will be prior to upload.  I've defaulted to my maximum amount of 10,000 which will be normal for some, but overkill for others.  The ability to change the number of rows/columns to accommodate the size of the sheet after loading, would be a nice addition.  

As a workaround, I'm destroying and reloading the widget instance after the initial load.

I want to be able to double click a column header  to set column width to max width of entries - autoresize

We'd need support for checkboxes columns in the new Spreadsheet control.

Currently, when working on a mobile device, the Spreadsheet doesn't have all the functionalities available when working on a desktop machine.  

Providing support for the merge cell, multi-cell selection  & context menu opening when working on a mobile device will be very useful for the Spreadsheet. 

Somtimes you just want to give the User a little Side Calc without a full blown Sheet, just the grid with some cells able to change.

 add search functionality to the Spreadsheet.

Set an indicator same way that grid already does but now in sheet cells when a record is 'dirty'.

I would like to be able to do conditional formatting against other columns and not just the existing conditional formatting.

For example:

Column 1 = 1.2
Column 2 = 1.5

If the value of Column 2 is greater than the value of Column 1, I want the color to be green.
If the value of Column 2 is less than the value of Column 1, I want the color to be red.
If the value of Column 2 is equal to the value of Column 1, I want the color to be black.

Dear Concerned,

I have observed that Conditional Formatting in Spreadhseet is not supported. Please add support for the same as it is very important feature of XLSX.  during financial calculation this feature plays an important role. Here is the issue that needs a fix.

1. Open https://demos.telerik.com/kendo-ui/spreadsheet/index

2. Check attached file CellColorFormula.xlsx, conditional formatting set for B2, C2, D2

3. Import attached file CellColorFormula.xlsx

4. Check Cells B2, C2, D2 and try to change values for A2, C2, D2

5. None of the conditional formatting is working

Define by Settings or via API Calls custom context Menu for any Cell,
like change current by Event BeforeContextMenu(ref ContextMenuElement)

Hide or disable Icons, Panes / Functions in the Toolbar

https://demos.telerik.com/kendo-ui/spreadsheet/index

1. Enter some text in a cell.

2. Increase the cell font size to 48.

3. Reduce the cell font size to 8.

4. Double click the row resize handler: the row height is not adjusted to correspond to font size 8.

The same behavior can be observed when opening an existing .xlsx file that has some text and font size set and following steps 3-4.