Typical Thai response to immediately tumble to large infrastructure projects like a third runway when the gap is that BKK was never designed as a transit hub in the first place (it was designed as an origination / destination airport only). Further, the anchor carrier Thai International seems uninterested in being a transit carrier … they focus on bringing people to / from Bangkok and that’s it.
Assuming TG wants this to happen (they’ll have to start behaving like Singapore, Cathay, JAL / ANA, KLM, Turkish, Emirates, and other serious transit traffic carriers), the core design inside BKK will have to change. Serious thought on how you efficiently hold and move people gate to gate while separating those originating / terminating in Thailand. Look at transit at, say, Schiphol (AMS) to see this done well. Heck, BKK can’t even manage moving people efficiently from international to domestic. Look at airports like Vancouver (YVR) to see how you do that well … avoiding the delays of requiring a new security screen while simultaneously separating the flow of transit / terminating passengers through separate immigration/ customs flows.
I think BKK could get this done if they would just focus on emulating what makes the world’s best transit airports work well (hint: it’s not runway capacity).