The already mentioned TWoTS is an excellent, I'd say the best, TFTD mod. (I suggest you get it from the official sources if you want to try it.)
TWoTS is quite a bit longer than vanilla TFTD, but not as excessively long as some other megamods like XCF or XPZ.
It is more difficult than vanilla TFTD, although the early game is quite a bit easier. It can still be completed with superhuman ironman if you know the mod well.
So while it's not really "hardmode" as such, I'm sure you'll find it enjoyable if you want a "TFTD+" experience.