Although this would be a nice feature, this is something that Riot has discussed in much detail.
Allow me to quote Riot_Mort here:
"ASKING PLAYERS TO PLAY SPECIFIC ROLES/CHAMPS
What hasn’t worked:
We knew this one was something we needed to be careful with. The example above can be completed by pretty much every role—except support. What we hadn’t counted on was how resistant some people are to playing their non-main position. This one had a lot of support players simply saying, “Oh well, I guess I don’t get to do this mission,” which was not at all our goal. We’re going to be really cautious about how many times we ask players to do this in the future, but we also have some improvements coming."
Source: https://nexus.leagueoflegends.com/en-us/2017/10/dev-missions-update/
We had missions similar to that during the Riven and Yasuo event. However, Riot claimed that those missions were incredibly unappealing to most casual players; so they reworked them into easier achievable missions. Additionally, they stated that they didn't want to make missions too difficult or force players to go too far outside of their comfort zone. Missions should be accessible to everyone, for the most part, and not feel like a chore.
If anything though, your suggested mission ideas should be applicable to those who have reached Mastery 7 on their champion, but they shouldn't be completely reliant on one's progress in a single game. They already claimed that they have something in the works for it, but it's not quite clear.