Spent 28 years as a protestant, and I think it's pretty simple: protestant has nothing to offer the Mass, in fact it's already ruined the Mass, but there are many things outside the Mass that the Church could consider:
1. Religious Ed: it's way underdeveloped in Catholicism. The issue lies in the fact that we want our kids to partake in Mass the same as us, which I agree is a good thing, a non-negotiable thing. But in the protestant realm parents send their kids off to Sunday school while they go to "big church" (that's what we called it in my family when I was little). It's efficient because the kids are getting their milk while the parents are getting their meat, to borrow the concept from 1 Corinthians 3.
The first step to fixing this is to stop Catholics rush out of the door once Mass is over, and worse in some abhorrently offensive cases, once they've left the communion line. Shame them for leaving early (I love
@Rhamiel's story about a priest who would shout that Judas dined and dashed too when he sees parishioners walking out the door after communion). Encourage people to stay at the Church longer. Offer more than just coffee and stale donuts in a janky multipurpose room for an after-Mass activity. Now with the parents staying longer the kids can go to Sunday school - have well catechized young adults working with young kids in a well-balanced curriculum of fun and education to teach them bible stories. Get the kids started young on the bedrock text of our faith: the bible. I have such fond memories of the old (cheesy) felt board with the attachable characters that were used to tell the biblical stories of Adam and Eve, Noah, Abraham, Moses, David...all those Old Testament stories that are so fascinating! If they're told well, if they're told right, they're better than any contemporary secular story someone might tell.
As the kids age they can graduate to introductory theological topics, then intermediate ones, they can learn about the Saints, the history of the Church, etc. Sunday school should itself be expressed as a timeline of salvation history. In the earliest years kids can sufficiently be taught that God created the world, that he chose individuals and groups throughout history to lead and to save his people until he finally sent his Son to save everyone once-and-for-all.
Anyway, you get the gist. If enough thought and attention is put into educating our kids and starting them young, that will resolve a lot of the severe doctrinal issues the Church is fighting today like that God awful German synod and the non-questions about sodomy and female priest.
2. Missions: short-term missions are a mixed bag. But the reality is we're getting our lunch handed to us by the evangelicals in developing countries. They go to places like Africa, build orphanages, deliver supplies, preach the gospel, get the Africa believers all excited and they become born again evangelicals and stray from the One True Church that Christ himself founded. It's not enough to have some religious orders on the ground with monasteries or friaries in a few major African or Southeast Asian cities and expect them to do all the leg work. Let's get our high school and college youth taking two-week long group trips to deliver donated books and clothes and hygiene supplies to orphan homes and also evangelizing them in the Catholic faith.
3. Fortify the Mass: Like I mentioned above the styles of Mass need to be parsed. People don't become Catholic to be more Protestant. I didn't convert because I wanted to see more of the crappy guitars and drums I saw in my old CMA church. I was lured into the Church by chant, sacred polyphony, incense, silence, bells, and the Holy Eucharist. So for the misguided people who think tambourines are a good idea in a Church sanctuary let's instead extract that from the Mass and offer Holy Hours in separate rooms outside of Mass times. It maintains a diversity that some people want and it relegates those other styles to their appropriate places. Honestly the darkly lit rooms with Kim Walker CCM lends itself well to the Holy Hour concept.
The Church needs to do a better job of separating these things out. They need to stop trying to make the Mass all things for all people and instead remind the faithful that they need to be the ones that conform. Conform to the Church conform to what the Mass was for thousands of years before the 1970s came along and the masonic infiltration of the Church was fully realized.
4. This ties into a larger problem but
begin instilling at a young age the concepts of duty and responsibility. That very little of what we do as Catholics is/should be about how we feel. It's great if our feelings match our responsibility that's not often the case. I rarely feel excited about chores but they're important. They build character. Well, all the same I want to build my Catholic character and we should be building parish cultures that stimulate every single member to do the same. The bare minimum is not good enough, "C's get degrees" is not a valid mantra in the Kingdom of Heaven. I'm sorry, but it just isn't.
5. Reintroduce a culture of shame. It's the lack of shame that has bred a society where sodomites, traneys, and baby murderers not only run rampant but parade their sin with jubilation. We need to bake into people at a young age why these things are abominations, and I think it's entirely possible to do so without feeding into the "repressive religion" narrative.
These are just a few ideas off the top of my head.