well here's the problem... picture you take out 2-3 champs late game... you ping madly DOBARONNOWLOLOLOL
So your team is doing baron, and suddenly, your little half-baron comes stumbling along. Suddenly, you're fighting Baronx1.5 Suddenly the other team can 3v5 you, ace you, and win the game right there.
Another side note: Riot before has said before something to the extent that too much randomness in a game detracts from its skill requirements. The better players get punished because lesser players could luck out and gain huge benefits. Having a random roaming minibaron adds a HUGE random factor.
tl;dr This is never going to happen, and you need to put a little more thought into suggestions like this..