The Brawler is the only TT variant hull at present. Now to be fair there's not that many other midlines that can be meaningfully Tri-Tachified, but humour me for a bit.
Brawler (TT)
It needs more! Instead of Insulated Engine Assembly, it should have Flux Coil Adjunct and Flux Distributor built-in so it can actually use lower-effiency medium energy weapons. And yes the Brawler is getting a flux buff in the next patch, but a variant reliant on energy weapons should still have better flux stats than a baseline ballistic.
Sunder (TT)
Yes it's basically a high tech ship already. Give it a blue paintjob, change the small ballistics to small energies, and make Advanced Optics built-in. Essentially this version of the Sunder is encouraged to use beams, particularly since it cannot use railguns to support the 700-range large energy weapons.
Heron (TT)
Change the medium universal to a medium energy and make Advanced Optics built-in. Yes it's almost the same change as to the Sunder, but it brings the Heron in-line with the Astral (which also has Advanced Optics built-in...). The overall idea is that larger Tri-Tach ships, if they aren't speedsters, tend to be more stand-offish. It's also an excuse to give a main Tri-Tach workhorse a blue paintjob.
As for other Tri-Tachable midlines...
Drover is the other major ship in the Tri-Tach lineup that can be painted blue, but there really isn't much you can do with it that would be meaningfully different from the base variant. Perhaps it could just have a paintjob and nothing else.
The Hammerhead, Falcon and Eagle are problematic to convert because they overlap too much with other ships in the same class. An energy Hammerhead would just be a weaker Sunder, and energy Falcons/Eagles would just be overarmed-yet-underfluxed Aurorae.
Other midline frigates (and also the Gryphon) suffer from the same problem as the Drover. Either there's nothing that can be changed meaningfully, or they're antithesis to Tri-Tach doctrine (the Centurion in particular).