Pretty basic concept that most people have thought of already. The one appeal for me is this would remind me of heroes ascent HA in guild wars one. Champion balance here won't matter since if people know somebody is using an IMBA champ they can gank somebody out / form an alliance until that champion is dead. I think rather than FFA a more of an appeal for me would be a 3 v 3 v 3. Although FFA does seem pretty fun both would be very fun.
FFA respawns shouldn't be there, so you're on the right track there. Games would probably only last 5-10 minutes lol, so this would be fun for people with a short amount of time. Either way both game modes would be fun
3 v 3 v 3 would cater to teams for a common objective and could encourage strategy to eliminate a less desired foe.
FFA would cater to those who do not have a lot of time and just want to kill a bunch of crap.
Both would be fun, new, and exciting.
, and you actually have to participate in fights, 
. Finally, no damage over time will be able to occur so no ties will happen, 
.
You will lose if your nexus is destroyed, you are slain, or you do not have the most cs of the alive players when the time runs out.
