I think the game will be well received by GW customers, so I expect we will see the other warbands sooner or later to be included in Shadespire.
And while Lizardmen and Seraphon don't fight for renown or treasures they certainly fight against anything which could bring benefits to Chaos, meaning they are ready to start a battle not to let Shadespire's hidden treasures being stolen by unworthy hands...
... so I personally believe they could fit into the game lore quite comfortably!
Regarding the game itself, despite all the positive opinions I've heard about it, it can't appeal to me.
I've never had a demo game, that has to be said, but there are some things which really put me down:
- I like having some tokens or miniatures on the table. But miniatures, plus tokens, plus dice, plus boardS, plus cards (of different costs, type, uses, icons)... Well, probably it's too much for me;
- The deckbuilding aspect is certainly challenging, but I'm the type of person who becomes bored after a couple of hours of it;
- Shadespire has been described as "competitive" since its reveal, and I frankly hate this word, this type of thinking and this type of playing... 'Cause to me "Competitiveness" often means "Desperate search of the unbeatable exploits that grant me a win" (I'm exaggerating a little);
- The probably only real, deep reason I cannot make me like Shadespire is the fact that in order to reach "balance and competitiveness" warbands of the same race have to field the same models (at least for now).
I know, the same warband can play in a completely different way depending on how its deck is built, but in reality (for now) every SCE warband is made by these 3 characters, every Khorne has those 5, ... and variety is furthermore forbidden by the fact that these models have specific names (not general ones like "Stormcasts liberator", "Ironjaw Brute", etc.).
All of it entirely IMHO, obviously.

I hope I'll change my mind in the future!