Ask around at places like Sonic Retro.

What about a game like Shining Force? I'd love to try my hand at making my own version.

If you're interested in making Sonic mods grab the latest disassembly from the GitHUB over at Sonic Retro. If you're wanting to make assembler modifications, I highly recommend checking out MarkeyJester's 68k tutorial and diving straight into editing certain parts of the code. For level editing, grab SonLVL and SonMapED will allow you to edit the sprites. Music hacking can be a bit tricky, but if you wanted to port music between various games check out the music hacking guide which is on Sonic Retro's wiki. Their wiki also has tons of tutorials on how to add certain moves, add extra characters in Sonic 1 and 2, etc.

Going to bump this in case there are more answers seeing as Google yielded unsuccessful results. I was actually intriguied by Pier Solar, since it was originally developed for Sega Genesis. Would be nice if the devs mentioned what program they used for that game. As far as google search goes, the 3 part dev kit one is interesting, however I prefer to program it for the PC and upload it on Steam as a new game than a modified mod. Also, thanks Ravenfreak on the info. Not exactly what I am looking for personally, but I guess it's a start.

Originally posted by DarkHawkGames:

What about a game like Shining Force? I'd love to try my hand at making my own version.

As it turns out, there is a feature rich GUI rom editor for shining force II. The download link is in the first post of this page:

http://forums.shining-force.co.uk/viewtopic.php?f=5&t=8022&sid=7770c3a5cf884d630fc251b4be57f209

Maybe we should have a topic of where to find our modding tools

https://www.youtube.com/watch?v=jBwSEAIFamw

The entire series is a good starting point.