Improve Access SQL
Access SQL is currently an extremely small subset of the SQL most other SQL dialects understand. The simple INSERT/UPDATE/DELETE commands are a good example, i.e. an OUTPUT clause would be of great help or many other syntax extensions well known from other SQL dialects.
Also the integration of a CTE like construct like with SQL Server would be a great step forward.
And Access SQL would also be much better if it would have support for XML output like with "FOR XML AUTO" in SQL Server. Querying with XML would be a little bit "too much" although not uninterestingly, but at least the possibility to create an XML string directly with SQL would be great.
Not to mention the SQL editor itself but that's often mentioned in other suggestions here.