...There's no equivalent to a Titleist iron.... ;-)

In many areas of the country, Titleist offers "Titleist Thursdays" where Titleist certified fitters will work with you outdoors with a doppler radar unit to confirm what you see visually with the club at address and how the flight looks. They will work with you on a best recommendation for head, adjustments, and shaft.

That said, if you do well with I-series irons, the fitter will look at how that works and likely start with a T100-S or T200. Not to say the T100 or 620 CB wouldn't be possible as well.