I partition Excel business use into three broad categories: Excel as dashboard, Excel as DB and Excel as calc engine. My own product - SpreadServe - is firmly aimed at the last; Excel as calculation engine. Excel functionality is so broad and diverse that it would be impossible to do a good job of all three in an MVP. There are a lot of small ISVs in the Excel space, and I think of them as falling into the following categories...
1. "Better" Excel: Resolver1 (now defunct), LibreOffice calc, ankh. Often they claim to be better at some subset of Excel functionality, but fall down on a lack of compatibility. There's a huge estate of existing biz critical sheets out there, and it's important to support as much as possible.
2. Vertical replacements: eg Anaplan or Tidemark. Often SaaS offerings, these aim to replace a specific application of Excel like budgeting.
3. Management & control: for instance spreadgit, Finsbury Solutions, Infotron. These aim to put version control, diagnostics, testing etc around Excel to mitigate the issues arising from uncontrolled sharing and changing of critical sheets.
4. Components for spreadsheet processing: SpreadSheetGear, kdcalc. Aimed at devs building Excel compatible server functionality.
5. Microsoft's own Excel extensions eg ExcelHPC, the recent Excel REST API, Excel Web Services.
IMHO it's important to consider all the competitors in these categories and how you're positioned compared to them. Especially the already mentioned spreadsheetweb, and Karma Platform too.