Keep in mind the only way to do what you're suggesting is to program a new game from scratch, there is no magic solution to keep the existing rules in place and tweak them as you want. (Yes there are some ROM hacks you can use to tweak existing games, but that's not the same scope.)
It's a lot of work, and for the 'dogs' you're talking about a lot of investment in time to try and get them improved. And not something you can really easily turnkey into something other people can easily upgrade with.
The FAST solution does offer that, which is why it's more attractive, you can potentially make "2.0" kits with it for people to relatively easily install.