I thought of this idea a few years back and it's implications.
1 - For this idea to work, pokemon need to be able to have 3 types. Not something gamefreak has ever hinted at. Let alone the mechanics almost falling to pieces with some pokemon potentially being 8x weak to certain typed moves
2 - The fusing pokemon must share at least one common type. If they had to be the exact same typing it would severely limit the amount of possible fusions and effectively make it a moot point.
3 - Would the resulting pokemon have one moveset, the other, a combination of both or all 8? and how would these moves be determined? There was talk previously about 6 moves for each pokemon being introduced in Gen 6, but the general consensus is that 6 moves make pokemon like Salamence un-counterable.
4 - The sheer number of possible combinations, new pokemon, stats, abilities, cries, footprints ect. It would be introducing a huge amount of data to the already data-overladden games. Not something that GameFreak would like to do as since most people wouldn't ever even attempt to make every fusion.
All in all, despite this being a really cool idea, the execution would be extremely difficult. Here's not to write it of though, but just don't hold your breath.