I have analyzed a lot of database modeling tools available on the market, and I see that they have many limitations and bad concepts. I think that yEd my be a good staring point to create simple and usefull tool which will be better.
First of all I'd like to see palette of symbols for database modeling. The minimum palette should have the following symbols: table, view, procedure.
Table shoud have the following properties: name, schema, owner and list of columns. Each column should have:
-type: text field with optional drop-down list of predefined types
-default value: text
and set of user defind flags (like unique, primary key, etc). This will allow to create models for any database type. This can be implemented as simple text field.
There should be also additional text field with optional SQL code, to be added after column definitions, to support database specfic features.
View should contain name, schema, owner and text field with SQL view definition. It could be nice, to present view as list of columns extracted from view definition (it should be simple, just extract comma separated values between CREATE VIEW and FROM).
Procedure should have: name, schema, owner, list of parameters, return value and text field with SQL procedure definition. It should be possible to extract paramaters and return value from SQL code for most of database dialects.
This set of symbols and properties should allow to model most of databases.
In next step you can add Foreign Keys as special relation where you can select columns from source and destination tables.
I've a lot concepts how database modelling should be made, and I can present them if there will be some interest.