I have no idea if someone else said something similar to this in this thread, so sorry if I'm beating a dead horse. But I think the main reason why we haven't seen any other trolls or Ursine characters is because they'd need a reason to be notably different from Trundle and Volibear. Trundle is the king of the trolls, the head honcho and (presumably) the strongest of them, too. If all trolls follow Trundle's lead, then where is the interesting story-telling? Now if they were to go with the idea of there being more than one tribe of trolls and Trundle's isn't the only one, THEN I think we have some potential.
As for Volibear, I think we run into a little more trouble, because the Ursine are part of the old lore. I'm not saying that they aren't a thing anymore, but we have no idea what the lore plans are for Volibear. I saw in another thread that Volibear, in lore, is supposedly strong enough to beat a Darkin on his own. That is no small feat, and that puts him quite high in the power list. I'm wondering if that's a hint that either A) The Ursine race is extremely powerful or B) Volibear is no longer part of a race, but instead some kind of nature spirit or elemental embodiment. Now if the Ursine ARe still a thing, then they still need to make said Ursine different from Volibear in a meaningful way, which they might be having problems with.
I don't know, I don't sit in at Riot board meetings. This is just my two cents on the issue. Now if they have new troll/Ursine champions in the wings that we just don't know about, that's great! If not, then maybe we'll see them down the road. Who knows? I do hope we revisit the Freljord soon, though. I don't think I remember the last next champ that got released from that area....