Your playgroup needs to be more creative, and for that you're going to need some inspiration. Has your playgroup tried swapping spellbooks with each other for a few games? What about alternate formats?
Part of the issue here is that when you're beta-testing a new spellbook that spellbook won't be as good and will likely lose more. Creativity hurts your competitiveness in the short term but boosts it in the long run. If you want to shake up your metagame with new ideas, don't be afraid to start a minor losing streak in order to build your spellbook.
Once your spellbook is good enough to outperform your opponents, they will be have to change their spellbooks or make new ones to stand a better chance against you. If your metagame gets stuck again, repeat the process.
Eventually if you keep making and testing new spellbooks, I suspect you'll have a predictable pattern of losing more when testing and winning more when you have a finished build, and your opponents will notice. I suspect this is the point where your opponents will really start innovating if they want to. Otherwise, it will be purely on your shoulders to change your meta.
Focus on strategies that you don't know whether they'll work or not. Regardless of what has or hasn't worked before, Mage Wars is so complex that I think I can safely guarantee that you can find something competitive if you think outside the box. But you need to test your ideas to know for sure whether they're worth trying from a competitive standpoint. Don't be too quick to abandon bad ideas. You might be able to figure out something to make them work and become good ideas. And if you really think your playgroup has exhausted all avenues of innovation, perhaps it's time for you guys to invest in another expansion. If they don't want to buy it themselves, maybe you could save up money for some holiday presents...
However, don't forget that if all else fails, new spellbooks aren't the only way to make your games more diverse.