"Your T-gauge models are great! Can you make (insert your desired peice of rolling stock here)."
Well, nearly always the answer is "Yes, but.." It's really a matter of time and investment, especially if the model isn't something I think I'm really going to want/need myself.
The total time and cost of developing a good model depends a lot on complexity and how well it would fit together with the existing mechanisms. While I enjoy the process, especially the research part of getting prototype photos and specs, it does take valuable time--which after my primary job and family, is not all that abundant any more. And money. I've developed expertise in the process but it still requires an expenditure, and nearly always there is some trial and error resulting in worthless "2nds" i.e. test models.
What I'm getting to, is if you really want me to make that model of "xxx", then consider commissioning it, i.e. subsidize the cost of R&D. It's probably not as expensive as you might think, especially compared to models in other scales. Past commissions range from undecorated shells, to concept-to-finished RTR models (see below Rhaetian Railway ABe 4/4 iii).
And if it's something like this, I might do it for a handshake: