25 champs at rank 5 would be a bit much. Riot has said they don't really want high barriers of entry to Ranked. Players who are not up to par are already handled; they drop to the bottom of B5 and then only match up with other very-low-skill players.
The idea of making you have champions of a few different types is sort of interesting. Although just owning the champs, or even having played them a few times, doesn't guarantee that someone will be any good at them (or ever willingly play them in Ranked).
If you can do that why not what I stated above? Let the trundles go 5v5 and the good people have a fair match!
Riot has discussed "Prisoner's Island" approaches many times. It is their opinion that it does not work very well for ambiguous things like "trolling". Other than a few rare behaviors that almost everyone agrees on (like intentional feeding), it is very difficult to define "trolling". Also, if you take marginal players (like ones who are negative towards their teammates but don't actually throw games) and put them in with the really bad ones, their behavior tends to get worse rather than better, making the overall situation worse. If you rely on, say, not getting reported in order to get off the "island", that will rarely happen -- the games will be horrible and everyone will get reported all the time.
They did compromise a bit with the ranked restriction system. Players who get reported a lot can be banned from ranked until they win a certain number of normal draft games, with chat restrictions imposed. In theory it forces them to actually improve their behavior and show they can work with their team and not be a jerk before they're allowed to play Ranked again.
They have considered a "Leaver's Island" for players who frequently disconnect/AFK/crash/etc. But apparently they're sticking with low priority queue for now.
