I'm a connoisseur of Megaman rom hacks, and I love Megaman 5 especially.
Here's some general advice -
In Megaman 5, the MegaBuster charge shot is overpowered, meaning for the most part enemies don't pose much of a challenge. Therefore, any challenge you intend for the hack must come from the level designs.
At all times try to keep the difficulty curve balanced. The robot master stages should be easier than the DarkMan stages, and the DarkMan stages should be easier than the Dr. Wily stages. AVOID sudden spikes in difficulty.
Design areas to make use of the Super Arrow! The Super Arrow is kind of like item 1 or the Magnet Beam, so don't be afraid to add areas where the use of the Super Arrow is needed to proceed or to bypass a section.
Always, always design areas with fail safes. If you include an area with a latter that you need Rush Coil to reach, then ensure that you're giving the player a solution if they should run out of Rush Coil energy - a spike, a pit, a respawning enemy, something.
Add an M-Tank to at least one of the Wily levels.
If you want to check out an example of a hack that gets MOST things right, check out Megaman Hyperion 5. This hack isn't completely finished (It's about 95%) but it gets almost all things right when hacking Megaman 5. I'm in contact with the author, and hopefully the issues with his hack will be ironed out before its finished.