I enjoy this in many games, but in a game that can already be sometimes hectic in danger rooms and arenas, it seems like a combo multiplier would be hard to do without creating more screen clutter, especially if you were attacking multiple enemies.

Also, how would something like this effect bombs? Swords are already the most powerful weapons, and it'd be a shame to further push the game towards extra sword-heavy gameplay. Perhaps you could boost the power of bombs in exchange? It would provide a nice boost to guns, as they would be easier to combo with.

Then there's the issue that each weapon has its own rhythm, and it wouldn't suit just to have one combo system. So it'd be a huge effort that the staff could be putting into new content.

Interesting idea overall, but I think I'd be against it. It works much better in a more detailed 1v1 combat situation, and in spiral knights, it seems like individual combo multipliers on multiple enemies sounds like visual overload. If it had been in the game to start with, it definitely would have made the game different to play, but such a radical shift would be a lot of learning for all of the players who are used to playing the current way.