I agree with most of the suggestions, though I think that for "balancing" Battlegrounds, a little more is necessary.
Part of the power of mechs comes from Magnetic as a tribe-exclusive keyword, with Annoy-o-Module and Replicating Menace effectively working as spells, making them more handy (and powerful) than Houndmaster for example. It's just icing on the cake that they work so well with Cobalt Guardian and Junkbot. Just glad that they didn't add Venomizer, Spider Bomb or Wargear.
They already said that they wanted to do balancing patches for Battlegrounds about once a month. So I am sure that they will get to that soon enough. But I hope they also take it as inspiration to develop unique traits for other tribes as well. Beasts are sort of established as "when you summon a beast", which is alright but limited to a few rare minions. For the most part, beasts don't work too well with each other, unless you get the right cards for them, while most of the Mechs do work with each other.
Demons normally have the identity of high power at low cost and a downside, but the only one that really does that is Vulgar Homunculus. Probably because Felguard, Pit Lord or Unlicensed Apothecary would be horrible and most other demons just don't work with Battlegrounds. The best demon-related card you can hope for is Mal'Ganis, and that sort of requires a Voidlord as well. Neither of them make demons really unique, and only late in the game.
About the only really outstanding ability for Murlocs is Toxfin, with no real gamechanger at the end (EDIT: Forgot about Gentle Megasaur, though it's too little too late, in my opinion). Maybe they could add Scargil as a Tier 6 minion, that lets you recruit Murlocs for 1 and thus gives you better chances for triples and more lategame snowballing. As it is, there really is no good reason to stick with murlocs, partially because their orignal strength of numbers and low cost is completely irrevelant in Battlegrounds, as all minions cost the same and developing wide boards can be done with anything.
So, balancing aside, I hope that Battlegrounds will give the developers ideas to just make tribes more interesting in general. Also, because most Elemental and Dragon synergies simply wouldn't work right now, and I hope to see a few more tribes and playstyles added to the mode in the future.