Ah, yes. XCOM! How could I forget that? I played the very first game on the Amiga of course ;)
Technically this genre has much replay value, because the development of yourself can be different and the response of the aliens can be different then, too.
And yes, Wyldermyth has many similarities to that genre. Now that you say it ...
But to the replay value again. I said technically because at some point the game is simply done, because every mission is the same: Fight the monsters.
So to keep the player busy with value I think the goals of the fights have to vary as much as possible. So a type of stories could lead to fights with a certain goal for instance. That missions (the fights) can be different I have seen in Druidstone - new game of the legend of grimrock makers: Lead a character out of the fight for instance and leave the monsters there ... way not?
Or solve a riddle while monsters are attacking and run away afterwards ... perhaps you already do that. I didn't play the last version yet.
XCOM of course has the big opponent too. First of all the big threat of the aliens. But in the end the big boss to be fought on the home planet of the aliens. I'll ride on that a little bit for a while ;)
The good the genre is and I loved it much so it can have a big problem of motivation too: If the own development does not run well enough the game gets frustrating more and more. And this frustration grows very slowly step by step. You can compare it to a slowly growing depression. So the problem is not the failure of the player but that it is realized slowly and very late in the game. This has to be avoided at any cost I think!
Wyldermyth can have that problem too if the management of the heroes does not run that good.
So no general book this time. Stuck at a game for a little seems better ;)