AFAIK, is not possible to develop a custom add-on or a plugin for Eve right now (for instance, a driver for an external database like Mongo, Redis, etc).
Is this a planned feature for a future release?
Do I need to write it with Eve or in TS/JS?
I wouldn't say it's not possible, but there's some work involved. You can probably put something together, but we don't yet have a protocol defined for getting data into and out of Eve, beyond just the JSON messages you can send to the server, which allow you to specify a change (records added and removed).
If you intend to build a native extension for Eve, check this document out.  It's a bit of a jungle at the moment (and improving that is on the todo list), but I'll be happy to provide pointers over on the mailing list if you're interested in pursuing it anyway.