Earlier quoted context omitted.
"I don't think this is a problem with LaTeX & "messy codebase", I think this is a problem with restrictions on iOS from Apple." No. It's a problem with LaTeX & messy codebase. Take Apple out of the picture, for a minute: the messy codebase remains a problem with TeX/LaTeX. It doesn't lead itself to easy extension, it doesn't lead itself to easy incorporation of features such as full Unicode support, Open Type font sp…
How can you say this when every item in your list is currently supported? Unicode, open type, pdf, etc. are all currently supported by new distributions.
I know a lot of the above are supported (I use LaTeX myself, and follow the developments quite closely) in newer TeX implementations, but not in clean way and not with first class support, to the legacy limitation.
I was asking for a clean implementation, no something like XeTex --that while supports several modern features it inherits tons of design problems from the original TeX.