Sounds fun... very similar to a chivalry game mode I had read about. Something where invaders are sieging a castle, and once they break into the keep the highest-scoring defense player becomes the king, who is then the target for the attacking team. That game mode alone made me contemplate purchasing the game... but alas it was not worthy of my 10 bucks.
Anyways, the only issues I can see with such a game mode would be:
1) Assuming it is set up like battle.
a) Cav could easily kill the target player.
b) If cav could be the target player... well, have fun chasing him down.
c) If you get an agi whore troll to be the target, and problems a and b are solved by not allowing cav in this game mode at all... then it would all be up to snipers.
d) Snipers. Able to kill the target in one shot from across the map. Sure, you can try and form a shieldwall... it won't work.
2) Assuming this game mode is more similar to siege.
a) Once again agi whores just kiting everyone. Also allows for them to easily suicide charge since no one else would be able to keep up to try and defend them.
b) Map abusing (one target can jsut sit on top of a long ladder/staircase or on top of a ledge/wall... or just any hard-to-reach place.)
c) Once again snipers. Xbows would be able to shoot even further if they have a height advantage.
This mode works (well, I am assuming it works.. never played) in chivalry since everyone is essentially the same few classes, and there is definitely better balancing. Anyone with throwing weps or archery is not nearly as OP as throwing in this game, or xbows who can 180-spin-headshot-without-even-taking-time-to-aim.