Gantt View / Timeline
A very basic Gantt Chart is mostly just a horizontal timeline with each task on a separate row. The horizontal axis can display daily/weekly/monthly time scales and scroll horizontally.
I’ll use the Month view in a Google Calendar to explain some ideas. Think about how often you are annoyed at having to click on “3 more” or “5 more” to be able to see the rest of the events for a particular day. Where is it written that all days within a monthly calendar view must be equal in height? I’d like to see a monthly calendar that had the option to either show one month on the screen, or expanded to a vertically scrolling calendar that did not hide any events within a “3 more” type of button. A variable number of rows, based on the number of events is not something that Excel or Google Sheets can do without scripting. However, this should be possible with Spreadsheet.com with a custom Monthly view. To see an example of the limitation associated with number of events per day in Excel, see this monthly calendar template.
Another idea for a monthly view is to think of it has a gantt chart or timeline that is wrapped. Instead of only showing a bulleted list of events for time-based events, you could use the width of the cell to represent 24 hours and display a small bar to represent the time and duration of the event that day. Multi-day events should of course span multiple days. However, if the start and end are datetime values, then it can display the true duration using the width of the day as 24 hours.
This is great for appointments and meetings and short tasks, and not as great for multiple tasks that span multiple days. The Week view in Google Calendar is a good benchmark.
In both the Monthly And Weekly views, being able to create new rows by clicking on the location in the monthly/weekly views would be a very cool unique feature of spreadsheet.com. If the start and end dates were manually entered in the table, perhaps you could modify the duration by dragging the edge of the bar.
Import / Sync with Google Calendar and/or Outlook
Having a table in spreadsheet.com that was linked to a Google Calendar, or would allow importing from or exporting to a Google Calendar would also be handy. So, developing with that feature in mind for the future would be wise I think.