There’s plenty of apps that have been built here -
but I’m not sure that a complete tool is possible yet, given the fact that large parts of Monzo’s API are undocumented & subject to change. Obviously that will no longer be an issue once the bank’s launched & the API’s stabilised (which will be necessary in order to establish integrations with partners).
Since Monzo have said that they’re not intending to build a complete interface in the foreseeable future, I’m not sure why they would choose to favor one 3rd party’s app over another. So I’d expect them to simply leave developers to build on the foundation that they’ve built.
There will be safeguards to ensure that developers can’t misuse Monzo user’s data / account access. Apart from anything else it wouldn’t make for great press if Monzo got that wrong & would get them shut down by the regulator pretty quickly.