Much like the killstreaks that you can activate in Call of Duty.
3 killstreak - UAV equivalent, some sort of hawk that flies over the map and might possibly reveal enemy locations
6 killstreak - Airstrike equivalent, some sort of goblin/dwarf/gnome aerial assault over a targeted area (similar to ult)
9 killstreak - Pavelow equivalent, a dragon spawn flies over the map and breathes fire over certain areas, causing an initial amount of damage, and then causing aoe dmg over the effected area
11 killstreak - AC130 equivalent, an earthquake effect begins and the earth at the center of middle lane cracks open, and you get control of a large demon/elemental for a limited amount of time. Can be used to push or genocide teams with ease.
15 killstreak- EMP equivalent, a lightning storm appears, and disables all items and destroys any gadgets/spawnable pets on the map ( wards, booboo, mines, etc.)
25 killstreak - Nuke equivalent, the map explodes with the force of a thousand suns, the game ends and the team who activated this gets an instant victory regardless of what the score is.
Other killstreaks could be instituted, or the killstreaks I have suggested could be limited to a smaller number based on balance and other factors. The effects I have suggested for the killstreaks could be tweaked based on design innovation and preferences.
Bad idea for HoN. Killstreaks already give the bonuses of being on a killstreak. Would make come back almost impossible, and have many games decided in the first 5 min (Like many CoD matches)
"Oh they have a pavelow, lets sit back and get farmed for abit"