I have said this somewhere before, but I can't remember where:
While M4M brings mutual benefit, it doesn't benefit the community as a whole as much because it is reliant on the other party as well.
Example: Person 1 needs to get his map modded but doesn't feel like modding, so he will M4M with someone and get his map modded and just write a halfassed mod.
Until such things change I will always recommend to just go mod maps on a whim. You can always help someone without taking anything back, especially if you mod more than you map. If everyone keeps doing this, you would kinda achieve a state where everyone is modding everyone's maps (which is equivalent to M4M), however there is much more activity.
Just some thoughts on this.
As for the maps, guest diffs should be fine, but it would always be possible to include a futsuu guest diff as well. The only problem is, people usually get so many guest diffs so if you map 3 diffs they won't be able to rank their map. This is why muzu+oni is still better than nothing. At this point it's possible to include a kantan to have your own mapset too, however there are 2 things that would be missed.
First one is popularity, since there are so many more standard players and they would download that mapset and they would get the taiko mapset with it too cause of the guest diffs. If you had your own set, the vast majority would just ignore it and move on.
Second one is the modding process. From my observations (I could be wrong) I noticed that when people map guest diffs, they don't care as much to get them modded, because the submitter is the one who is getting mods. Since you would have to get mods on your own mapset, some people would get discouraged and demoralized (I dunno why but you can see this is true by the discussions happening around, people who are complaining they have to map more to get their guest diffs ranked for example).
Basically, there will always be problems, and until people just start modding without special conditions, nothing would get solved in my opinion.