Ever notice how Castlevania and Ninja Gaiden on the NES are basically the same game? The life meters are similar. Levels are timed but rarely is running out of time a concern. You hack away at regularly placed floating objects to collect power-ups, including a variety of secondary weapons and the ammunition to power them. Enemies knocking you backwards off of a ledge to your doom are the single greatest annoyance and a heavily exploited element in the level design. Both have kickass music.
I want Konami and Tecmo to get together and remake both games with Ryu starring in Castlevania and Simon starring in Ninja Gaiden. Have them bring their respective weapons with them. I think Ninja Gaiden would get easier since Simon's whip has good reach. Ninja Gaiden is a bit harder and faster paced, so I think this would help balance both. Of course Simon would have to gain a wall jump as well. Let Ryu keep his too; though it probably wouldn't be put to much use, there might be times it is exploitable.
And then, for shits and giggles, bring Capcom into this and put Arthur in both games.
**Posting From My Wii is a (likely) continuing series in which I jot down idle musings on small subjects, the idea being that since I am in fact posting from my Wii I a) cannot upload pictures, and b) cannot and do not want to write much.**