Stegadons are good units. They're just overpriced a bit. The fact that they count as battleline and not behemoths in Thunder Lizards means Thunder Lizards has some interesting options. If Stegadons come down in price, the subfaction could be pretty good.
That said, I don't think it will ever take the form it used to. I could easily see it have a Slann and a Troglodon (for it's -1 to hit aura, unlimited range vassal casting and skink spells), and some Saurus Warriors to keep it balanced. But bringing 4 to 6 monsters is a real possibility.
Diversifying the kinds of monsters will maximize the number of Monstrous Rampages you can do. Bring one of each and you can potentially use all four standard monstrous rampages and all four Coalesced monstrous rampages in the same charge phase.