Pichu is an icon

I find it cute that we'll actually get it in the wild and have to evolve it. Hasn't been that way since... Sinnoh
There were two major suggestions for the free space besides new Pokemon; Rattata or Spinarak. Rattata due to Yungoos being brought in to deal with a certain Pokemon and historically that was rats. And Spinarak because we havent seen the Pokemon yet and it could be a night-time exclusive only Pokemon - and could hint towards time of day exclusive Pokemon.
The last generation to include time-based encounters was Gen 4, and if we get Diamond Pearl remakes as expected, it would only make sense for Gen 7 to have them too...

I'm kinda leaning towards Spinarak - or a new Pokemon that only shows at night.
1-9. Starters
10-12. Pikipek & 2 Evos
13-14. Yungoos & Evo
15-17. Pichu, Pikachu & Raichu
18-19. Spinarak & Ariados
20-21. Ledyba & Ledian
22-24. Caterpie, Metapod & Butterfree
25-27. Grubbin & 2 Evos