Hovering or clicking on the series name in the legend will highlight or toggle the visibility of the series. 

Currently PieChart exposes the labels of its items outside the Pie, while the DonutChart exposes the labels inside the Donut.
 Add the ability to choose the location of the item's label (e.g. center, insideEnd, outsideEnd) in PieSeries and DonutSeries. This can be done either by exposing additional property or by expanding the values range of the current position property.

I would like to be able to supply DateTime values for the x axis of an HTMLChart scatter line series to represent a timeline.  Would it be possible to make this easier by having the control plot points plotted based upon the tick count of the value and add support for appropriate label format strings?

The axis is numerical, but it could have a DataLabelsField property like the x-axis so text for each label is taken from the datasource (a label per datasource row). Then the numerical values (YValue) of each item can be rounded to the nearest integer and if that matches an axis item's index the series item will correspond to this axis item.

Currently BarSeries and ColumnSeries can be stacked in only one stack. Add the ability to group the stacked bars/columns Series through additional property (e. g StackName) for each Series in different stacks.

The event should be similar to OnClientSeriesClicked event. It should expose information about the clicked series.

For the time being you can stack 100% series through the chartObject. For example:
ASPX:
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function pageLoad() {
				var chart = $find("<%=ColumnChart1.ClientID%>");
				chart._chartObject.options.series[0].stack = { type: "100%" };
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:ColumnSeries>
					<telerik:ColumnSeries Name="Product 2">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="35000" />
							<telerik:CategorySeriesItem Y="13000" />
							<telerik:CategorySeriesItem Y="20000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

Rejected with the following reason:
The purpose of the RadHtmlChart is to visualize data and not to manipulate it. You can manually group a data source, so that its structure is suitable for a chat data-binding. Such an example is illustrated in this CL - http://www.telerik.com/support/code-library/group-radhtmlchart-data-source

provide htmlchart in 2D/3D