SAND, a newly released app from developer Matteo Caldari, does just that. It's a grid-based sequencer that'll host, sequence and record your audio plugins on iPad and iPhone. The user can create an unlimited number of tracks, each of which contains a MIDI step editor and space for multiple clips. Plugins can be inserted into each track and opened up for control via their own GUI, while parameters can be individually assigned to knobs within SAND's interface for easy access.
MIDI mapping is available in SAND, and every function in the app can be controlled via an external controller through customized mapping. Caldari demonstrates in the video above by using a Korg nanoKEY Studio to remotely control SAND.
SAND has only just been released, so it currently lacks a few key features one might expect, such as the ability to work with audio clips and automate plugin parameters. However, Caldari has shared a development roadmap that outlines plans to include these capabilities and many more in future versions. You can request features that you'd like to see, too.