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.

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.

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

On excel, you can filter merged rows. On the kendo spreadsheet widget, filtering on merged rows is not possible. It would be a very useful feature for us. 

Also worth noting is that if you convert an excel file to JSON using Telerik RadSpreadProcessing, and load the kendo widget with the JSON, the widget displays a filter icon but clicking it loads a non-functioning filter. Wouldn't it make more sense to load with a message along the lines of 'You loaded a spreadsheet with filters applied to merged rows, which is not supported in this widget'. Otherwise it's quite confusing to our users.

Enable Cut/Copy cell values and can Paste it in mutiple selected values

In Kendo UI spreadsheet when wrap cell text it currently split text in middle of word. Change so that splits on white space

E.g. italian culture uses ";" as parameter separator in formulae.
Our client will adopt spreadsheet component on a global basis and users need to be able to e.g. copy/paste from excel to spreadsheet component using their culture

Additional functionality for:
-- Conditional Formatting -- coloring of cells, for example based upon a value and then be able to import from excel, and export back out.
-- Data Validation (dropdowns).  Also be able to import and export back out.

It would be great if you would add accessabbility to the spreadsheet as it seems to be the last control without this feature.

Currently the focus gets trapped inside its grid and the user cannot leave the spreadsheet with the TAB key. At least this should be avoided. It would be also better if only the first cell would be focusable and you then could navigate with the arrow keys, as with thousands of cells it is not very comfortable to use the TAB key.

Ability to hover over a cell and see a tooltip

In the scpreadsheet you can use controls like  dropdowns & datepicker , but should be helpful if user could to customize the design and behavior. 

For example, the dropdown and any other control needs be opened since an icon at right that is displayed over the next column, As user i would like to choose how i want to be displayed the control: Open clicking on it, set expand direction, styles, etc.

Allowing custom editors to be inline would be very helpful. In particular, inline editors using other Kendo controls (e.g. masked textbox, numeric textbox) should be possible.

Currently the font sizes in the Spreadsheet are in pixels, while in Excel they are in points (pt). The values for the fontSize remains the same upon export / import. As a result, fonts in Excel appear lager than those in Spreadsheet.

Currently, after export to excel and opening the exported file again in Spreadsheet, the row heights for rows with predefined height change. The above could be easily observed on the Index demo:

* initially, the height of the top row is set to 70px;

* export the Spreadsheet to Excel;

* after re-importing the above file, the height of the first row "Invoice #52" becomes 81px without any apparent reason.