I still can't quite get over the awesomeness of cross-generational evolution and will use every opportunity to fawn over it.
Some Pokémon are just screwed out of luck in terms of viability. Whether they were designed to be weak to begin with, lack any distinguishing features, or were left behind by power creep, we tend to overlook them when picking our teams. Sometimes, one may even think "yeah, I think this Pokémon looks sort of neat, but why would I use it when I could use this instead?"
And then ... cross-generational evolution comes to the rescue.
It gives many Pokémon an entirely new lease on life. I love it when my perception of the evolutionary family's first stage changes from "this evolves into some dex fodder, I'll skip it" to "this could legitimately become a really good Pokémon eventually, I have to catch it!". Even if the final evolution is tedious to obtain or arrives very late (here's looking at you, Hydrapple!), I'll still happily catch and use the first (or the first two) stages, due to the potential they now exhibit. Sure, Pokémon like Primeape, Piloswine, or Pawniard didn't get any stronger on their own by gaining an evolution, but the promise of future Annihilape, Mamoswine, and Kingambit make that middle phase a lot less awkward.
It's fun to look back at old Pokémon and see what wonders were worked through cross-generational evolution. Relative weaklings like Golbat, Yanma, and Togetic gained powerful new forms. The likes of Electabuzz, Ursaring, and Piloswine were no slouches originally, but their evolutions are beastly. Pokémon like Porygon and Roselia were saved from an obscure existence as single-stage dex fodder, and now serve well as steps on the road to greatness (granted, they had their fans who dislike seeing them being bypassed as mere transitional stages on the way to better Pokémon). And some Pokémon even received pre-evolutions to great effect: it's easier for trainers to fit a Magmar on their team when they catch it early on as a Magby, when their team of Pokémon is not locked in with a full set of six yet. And branched evolution can make a Pokémon more interesting too, look no further than Eevee or Poliwhirl.
Likewise, it's fun to look forward and try to imagine which Pokémon we currently consider boring and underwhelming, but will become great first and middle stages on the way to more powerful evolutions in the future. Many fans keep their "Pokémon that need an evolution" lists, and cross their fingers every time a new game is on its way: will this be the game that redeems my favourite, underwhelming Pokémon? I know I applauded wildly for Girafarig and Stantler in recent games.
I also like it when cross-generational evolution gets to span an evolutionary family of three, because this allows for the first and final stages to differ a lot more than two-stage evolutions do. That means cuter first stages and more badass final stages (both looks-wise and mechanics-wise), because the middle stage can serve as a transition between two extremes. In turn, the cross-generational evolutions tend toward the badass side, because there's already a second stage to build further upon. For instance, the cute Teddiursa already evolved into the big bear Ursaring, which gave Ursaluna the opportunity to be a great hulking brute of a bear. Swinub is a cute little piglet that evolves into the hairy boar Piloswine, then Mamoswine could be a freaking mammoth. And they could go all out with cuteness when they added an evolutionary stage before the elegant Roselia.
So I'm pretty stoked for Legends Z-A, if it does anything like Legends Arceus did. I really look forward to see if we get an equivalent of Ursaluna this generation too. I just hope they don't overdo it with "regional evolutions", because while I really like Pokémon like Sirfetch'd, Overqwil, or Obstagoon, they did nothing to improve the base form of the Pokémon they evolved from.