Remathilis
Legend
The problem with that line is the price point. If you don't push it as a mini, but as a D&D action figure or whatever, you can sell it in more places.
Imagine a boxed set with a Gargantuan Tiamat and little action figures of Hank, Sheila, Presto, Eric, Bobby and Diana! Not unlike the recent Transformers Scene Packs that have human minis packed alongside the (actually transforming) robots:
Guess who makes those: Hasbro.
That's a bit of an apples : oranges comparison.
The Transformers scene packs have a lot more inherit value as an action figure. It can transform, and therefore has movable positionable parts. It also have a huge mega-million dollar movie and cartoon series to springboard off of, as well a enough name recognition in that 6-13 age demographic to make throwing in the figures candy to Yet Another Bumblebee.
I don't see a fair compassion to making a Tiamat with figures. The D&D cartoon is only known to a small group of devoted fans (and reviled by half of them) and (as I've said before) the amount made and expected amount purchased doesn't meet the high volume needed to make it at the same functionality and value as the Transformer scene pack.
The closest to what you're proposing would either be the Star Wars AT-AT for the SWM line or the Icingdeath/Drizzt Icons pack. Neither, AFAICT, sold well enough to warrant further expansion, and that's coming from one of the biggest merchandising juggernauts (SW) and some of the most famous D&D novels (Drizzt) of all time!
Sadly, unless The Rouse or Logan come out and tell me there is a new D&D Icon coming out, I'm going to assume your not going to see any further Gargantuan or Colossal minis.