What makes a good new hero? This is probably a very subjective topic but I think there are certain things that make a new hero interesting and add something to the game.
#1 Team orientated.
This doesn't mean a support hero but the best heroes are ones who can be part of a team. These types of heroes can come in many shapes and sizes. In general team orientated heroes either support, stun or deal a large amount of aoe damage.
The classic team type heroes are Crystal Maiden(glacius), who has an aura to increase mana regeneration, Tidehunter with his massive aoe stun (in DotA) and Undying(DotA only) who can slow and increase damage taken of a large area.
#2 New Mechanics
There are lots of cool types of mechanics, even moreso in HoN, available to a hero who adds something that is different is always very nice. Some of my favourite DotA mechanics are Vengeful Spirit's (DotA only) Nether Swap which swaps your and the target heroes position, Admiral's (Dota Only) torrent which is places on the ground and takes time to 'erupt' and most of Clockwerk's (Forgot his name in HoN) moves. Rocket, Power Cog and Hook are all great.
The most interesting mechanics are unique and powerful. In order to tame that power it is best if the mechanic takes skill to master. This brings me nicely on to my final point.
#3 Skill based
The best heroes take skill to play. The archetypal skill to play character is Priestess of the Moon(Coming soon to HoN) who in the right hands can stun for 5 seconds, deal a huge amount of 'nuke' damage as well as remaining strong into the late game.
To use Vengeful Spirit's nether swap as an example there are many uses for this besides swapping an enemy towards you. You can swap an enemy who is channeling a spell to interrupt it, you can swap teammates out of danger or who have a projectile e.g. Storm Bolt coming towards them.
Moves that have a lot of potential which allow for the occurence of 'good' play, search youtube for Mirana Arrow, are best.
To conclude. If you are coming up with a new hero try and think of a unique mechanic for an ability or set of abilities that are team focused and difficult to master but provide a lot of strength when used correctly.