Add built in vba parser and interpreter so macros run properly. (Just a bit more time consuming than an ASM interpreter.)
I think excel mobile could (and should) run macros. I could build a VBA interpreter for you...I've done a full ASM interpreter in several languages on codingame.com, it should be pretty easy to make one for VBA just way more time consuming. I have certificates for about 30 programming languages and have experience in about 20 of those.
Peter Stahlecker commented
Why don't you do it, and sell it on the App store??
If it allows EXCEL macros to run, surely many people will pay money to get it!