you could certainly use an ipad to sequence.
Depending on what you're going for, there are a handful of options
• Nanostudio 2 - daw-like piano roll, with a bonus sampler and pretty impressive 3 osc synth
• Xequence 2 - similar, but without the option to run instruments and midi plugins, of which there a lot of good ones
• Polyphase - fun 4 part generative sequencer. Perfect for the PFM2, but not perfect if you want predictable results.
• Atom - A piano roll Auv3 that you can run multiple instances in any host. For the most modular approach, I'd suggest AUM as a host running Atom. there are fantastic midi fx plugins such as Mozaic, midifiltr-PG, midLFOs, cykle, Cality, Fugue Machine, etc.. as well as lots of drum machines and such. These plugins can also run in a host such as Nanostudio. Sadly Nanostudio doesn't have audio recording capabilities just yet.
I personally prefer a hardware sequencer as it's more hands on, but I sort of use a combination. I'm using a Squarp Pyramid, but all midi and audio goes through AUM for audio and midi processing. Nice thing is that AUM also doubles as a multichannel audio recorder!