I am running an RME Fireface (8 analog in, 8 analog out) with a Tango24 (8 analog in, 8 analog out) total 16 ios...although not Myteks, it hangs pretty well with the much higher priced stuff...adding another Tango will give me a full compliment of 24 (sidebar: the Tango, to my ears, sounds a WHOLE lot better than you would think...clocked off the Fireface, its a wonderful unit, and they dod pop up for sale here often...)
I would think the next step up would require a revamp...one Lynx Aurora 16 and one Aurora 8 with an RME AES card will give you 24 great I/O...at the higher sampling rates if desired...
You could also look into using an Alesis HD24 with the upgraded convertor card (search for the Jim Williams mods for details)...this would give you the option of running really good ADDA and having the an automatic HD backup.
If you wish, you could upgrade your MOTU with a pair of their HD192...I think anyway you look at it, you are gonna spend come cash to significantly upgrade...I looked at all the options, and the RME/Tango route met the price/benefit route for me at the time (although I am strongly drifting to the Lynx stuff for the next upgrade)