Kind of already did limited fusions between Kyurem, Necrozma, Galar fossils, Bill (Kanto), and Lusamine (Alola). Assumes you mean between any two Pokemon, though.
Pulled it off fairly well in fangame Infinite Fusion. Feels pessimistic on the Pokemon Company doing a good job. Deals with concerns that fangame did not, however. Examples:
1. Balance. How do you balance that many Pokemon without making the base form bland in terms of ability, moves, and maybe aesthetic design?
2. Art. Needs to make all the Pokemon fusions look halfway decent. Feels like a monumental challenge to produce all the possible fusions for a small, 100 Pokemon generation. Used a generic method in Infinite Fusion for anything without a custom sprite. Created some strange-looking Pokemon. Doubts the Pokemon Company would allow that. Only grows worse if in 3D. Almost certainly sacrifices any kind of Pokemon movement.