Why not make a new class?
The Goblin is the new class, it just makes Creeper redundant. Games are meant to be 45 Minutes to an Hour long, the average changes depending on the map. Maps that favour Dwarves (Like Delphia, Grapevine, Nolrim) persistently exceed this time and get a large number of kills, this is mainly due to the latter parts of the maps allowing for extensive and defendable Proc Halls to be built. If Goblins sole purpose was destroying defences then these maps would be hitting the average. And you are forgetting that the Goblin update will happen in tandem with a range of other updates so it will be balanced around that rather than the current game.
You know who gets all the kills at the end of the majority of games? Creepers.
The majority of creeper kills are on players that would have died regardless. Most Creeper players target bleeding Dwarves who probably would have died to other Mobs anyway. This is more likely at the end of a game when the gold has run out and more players die from broken armour. I'd argue that if Knockback creepers were removed, you'd actually see more players dying. In the rare instance the KB of a Creeper blast puts a Dwarf in a difficult position you'd probably see 20 other instances of a Creeper launching a Dwarf in a direction beneficial to the Dwarf. Not to mention the Creeper Damage is only enough to remove the absorption hearts from a Procing Dwarf, which can quickly be recovered as AI's are affected by KB as well and mostly go in the same direction. And by far, the majority of kills in a game is still held by Zombies.
If you combine the two then it will be OP and unfair
I believe the intention is to remove the knockback and reduce the damage of the Goblin, this is to shift the focus back onto destruction.
When was the last time you saw a game going over 3hr? Or even 2hr?
With the addition of the Side Rotation the chance of 2 hour long games went up significantly. Back on PMC there were probably more 2 hour long Nolrim Games then one hour ones. And my only 3 hour game was on H'Cet War, and since that is included in the side rotation it is a possibility. I've already had some 2 hour Nolrim Games. And it isn't even map dependent, all it takes is some good Heroes or a good set of Dwarves and the game time goes up significantly. DvZ has always been balanced around larger games, this means that good games should be closer to the average then games with less than 50 players. Playing as a mob can become tedious even in an average game, games that run over an our just become boring as you get stuck on one section of a map for excessive amounts of time because there aren't enough Destruction Creepers removing their defences in order to progress.
I think that the gold situation already heavily favours the mobs.
The gold situation is entirely Dwarf dependent. If players don't gather a lot of gold at the start of the game then it will run out, If they don't efficiently manage gold usage then it runs out, if they aren't scavenging as much gold as they're using then it would run out. This is no different to building walls. If they don't build a forward wall they can't hold first, if they don't build PROC Halls they can't hold that section of the map. Exhausting gold supplies and removing defences is more important then getting kills, because at the end of the day if good players are the only ones still alive and they have a lot of gold and good defences they will hold the shrine as long as they like. Mobs that specialize in these factors are needed.