From Chainmail to D&D Next
I played some Palladium back in the 1990s, most Rifts, Robotech, and TMNT. I remember thinking the system never ran smoothly, and was hard to read and understand. I don’t current own the books anymore, but Jim does so I thought I’d list it as an option.
Palladium is class and level based game, with a d20 mechanic. That is where the similarities with D&D end. It attempted to make combat more realistic, focusing on how armor works. The system once internalized could play fairly quickly and allowed for a wide range of characters.
Play Palladium if their source books interest you, or you are looking for a break from D&D without moving to far.