I just saw an email from Affinity that their vector illustration software Affinity Designer will be coming to the iPad next year 
Just re-visiting this thread to update now that Designer is out for iPad Pro (don't think it'll run on any lesser-specced kit, plus I can't really see the point without the Pencil).
As mentioned in previous discussions, Designer is the only credible (IMO) Illustrator alternative… if you don’t need to supply live AI files to any of your clients, then you can already definitely letter using the desktop version. There are people doing pro work with it already.
I’ve downloaded the iPad version and had a quick play. You can set up comic size pages in CMYK, place art, add layers. Type controls seem good, including Opentype options. Autoligs work by default. Art, fonts, etc, can be pulled in from iCloud/Dropbox — I copied a selection of .OTF fonts to a Dropbox folder and the app installed them fine.
Default save is local, but iCloud Drive is also offered. You can export to iCloud/Dropbox/Files in all the important formats. Export options are extensive, and include EPS, PDF, TIFF and PSD.
Initial verdict: I could absolutely letter on my iPad using this.
(It's looking like this, Affinity Photo, and the very impressive port of Clip Studio to iOS has finally got Adobe worried, since they've now announced a full version of Photoshop is in development for the iPad.)