Requiring JavaScript developers to use the Notes, Designer or Administrator client will force them to install an additional (unnecessary) IDE. This means post-V10 it's time for a radical re-think.
JavaScript developers will likely be coding in Visual Studio Code or Atom, using a JavaScript API to access their NSFs. All relevant functions (database property management, form creation, view creation, relevant admin functions) should be extended to JavaScript as additional npm module(s).
XPages is (I think) the only aspect of pre-V10 app dev that requires Eclipse. There are a number of DXL round-tripping bugs with traditional design elements, so I would be surprised if anyone is actively using source control against traditional Notes Client (or traditional Domino web) applications. The big benefit of the Ecipse client is a much more powerful designer search function. Develop a competitor for that in the R7 Designer client, make it standalone, and you have a valid designer client for traditional Domino development that is likely to address performance criticisms of many developers.
Investigate the open source work on XPages editors in core Eclipse, Forms and Views, and NSF ODP tooling projects (all are Apache-licensed I believe) and XPages developers can potentially develop their applications in standard Eclipse by just adding a "Domino XPages" plugin that provides the relevant editors and builders. This should also be combined with the recommendation to separate XPages UI from data NSFs, which has become a best practice for many XPages developers already. The data NSF could be developed, if required, with the traditional Designer client. If necessary, as most of the work is done, work with the community to take their work back into the core or make this a community client, advocated by IBM / HCL.
Moving idea to product : Domino Designer