A few years ago now, I started reversing Head Over Heels. It was quite a bit of fun, but eventually I stalled. I've done incremental work, but really wanted to make a bit of a push.
What's really got me unblocked recently is to process the labels in the code and build a call graph, visualised using dot. While I could poke around before, looking for likely chunks of code to reverse, a call graph makes things much clearer - it's obvious which subsystem a chunk of code ties into.
From there, I found that the best approach was bottom-up - identify what the small chunks do, and go from there - and I think I've finally pulled apart the full structure and all the functions, even if I haven't got all the details sorted. Currently it stands at 13K lines of assembly, including comments and blank lines. Data gets added on top.
I think I'm pretty much there. The first 80% is there, and I just need to get that last 80% of polish in place to have a nice, clean fully-reversed Head Over Heels. Fun, fun, fun.