Firewire seems to becoming less and less popular on computers these days. Moreso with USB 3 around the corner. Some new top-end laptops don't even have a firewire port anymore.
Reason I think a TI|3 might surface is that the TI|2 was a refresh of the TI|1. The TI|2 didn't bring any new hardware functionality per se, other than the 25% extra DSP. I remember Marc saying the TI|2 will never go anywhere the TI|1 wont, which would confirm the "refresh" theory and also indicates the TI|2 is limited to the lifespan of the hardware and software capabilities of TI|1. The original TI keyboard was originally released in 2005, and the TI|2 in January 2009 (two years ago this coming NAMM).
If Access wanted to employ something like user-custom waveforms in a new OS, which has always been a popular request on wishlists (mine too) since the original TIs were released, I don't think they could do something like that with the existing TI hardware (otherwise they would have done it). I can only assume there's no physical, usable flash storage capacity to store them, and would need new hardware to implement it (if ever they chose to do so).
Those who bought a TI|1 at OS version 1 have effectively had four brand new keyboards, one for each and every new OS version. Since OS v1 (which was already a substantial upgrade from Virus C), Wavetables, Formants, Graintables, Atomizer, all the effects (new Characters, Tape Delay, Distortions, Ring/Tremolo modulation, FrequencyShifter), Side-chain, Stomp boxes, Comb+Vowel filters, step sequencing, 64-bit support, etc., have all been added with new updates.
I wonder when the physical hardware limits will pose a barrier to new functionality.