Also, not only providing new tools, but also making battles larger in strategy games. The type of challenge is different, from a handful of units that you have to use judiciously to a large army that might be cumbersome to maneouver with. Depending on the mechanics and the interface, some games are good at one of these but not at the other. Some choose from the beginning to be a small army or a large army type of game. Not all the games that go from small to big manage to do both well.

To follow the comparison with your post, in those cases, mechanics or the interface fail to make playing with the larger army a better experience than with the small one, or mechanics do not make playing with a small army enjoyable (although the story, maybe with dialogue, cutscenes, scripted encounters etc might come to the rescue for that part of the game). When a strategy game can do both (from Command & Conquer to Starcraft), it feels really nice.

The Messenger starts as a regular retro platformer in the first half then it opens up as a metroidvania. One of the best games in the genre in my opinion, loved it.

I'll add a couple of visual novels that I enjoyed from start to finish but arguably have better second halves than the first as that is when the plots really start rolling.

Utawarerumono Mask of Deception starts off pretty slowly (mostly to introduce characters and build up the world) and then kicks off properly in the second half of the game. The sequel Mask of Truth could fit this too with it being a better second half to the first games story though both are great.

Steins;Gate also has a slower start up, particularly with the initial scientific expositions, but once it reaches that point (if you've played it or seen the adaptation you'll know which point I mean) it really speeds up and leaves you wondering what will happen next.

Sadly, none of these are actually available right now on GOG as far as I know.