Depends on the market you are aiming for. In your place, my first decision would be whether to link it to a specific RDBMS implementation (which would be Oracle, SQL Server or mySQL) or to try to make it flexible to use amongst any of the major databases. The latter would obviously add massively to the complexity (development, testing, support).
The second would be the OS platform. Something that is specific to Windows (or perhaps Windows Vista/7 upwards) or can be used on Linux, OSX etc. 32bit/64bit would also be a consideration here. I'd need to know that it can run on 64-bit Windows 7 at least.
Thirdly, what do you develop your Rapid Development Tool in. Dot Net would pretty much be Windows only. Java or Python offer more choice.
Fourthly, are you making this as a commercial product, or as open source (or both). Bluntly, if I was looking at buying such a tool to develop my XYZ product in, I'd be looking at long-term support. If the tool doesn't have a five-year pedigree of ongoing support, I'd be very hesitant and would only consider it if it came from a company of the likes of Microsoft/IBM/Oracle/Apple with some serious history in the field. And even then I would be skeptical (search on Microsoft's Oslo/Quadrant/SQL Server Modeling).
I'd also be looking for professional tech writers for the documentation.