Well, i think that the "most difficult planets" (glacio = hard, atrox = very hard) are not that hard really, so i think that a good idea would be add 2 different suits for each of those planets, for glacio the Heat Suit (or Anti-Freeze Suit or something like that) to avoid the cold that could kill you slowly, but it will kill you, and the Radio-Hazard Suit for atrox, to avoid a very strong radiation in the surface of the planet, this radiation without a suit could kill you quickly.
I don't know if this is a requested post but i'd like to add that for more difficulty because you can't really notice a real difficulty addition except for gateways (requires more energy) and the enemies.