Originally, before getting revealed as "evil mastermind" he could use her vampiric charm to make his plan in motion, it would end with him puppeteering behind the scenes and Euphyllia wouldn't be involved in politics since she wouldn't sit on the throne beside him, which would hinder her growth as they planned to use her as just a poster girl.
But sister revealed that instead making it a private matter and, here we are...
There are SO MANY other things he could've done, but he didn't, did he? He's a bloody idiot. Other workable methods and actual plans: (not lol me luv sister so much me never talk to her again and try to get her to kill me or do an em derp uprising for no reason)
Just off the top of my head, here's a few things Al could have done:
A) Taken the throne and brutally crushed anyone who opposed his reform ideas. There are several ways to accomplish this as a monarch. What do writers think a monarchy is, anyway? As king, you rule. Make an example out of anyone who opposes you. Use political marriages, take hostages, play the damn Game of Thrones for god's sake. Ironically, if he had been
more of a bastard, he could have been much more effective.
B) Actually
talked with his sister about his concerns. Which he did not do and did not even seem remotely interested in doing. Oh no, he's just a massive ball of insecurity, inferiority complex, and ego. They could have worked together, but no, they don't understand each other and don't talk to each other.
C) Instead of throwing away Euphyllia he could have used her to get the nobles on his side in a much more natural and less 'magical brainwashing/dangerously unpredictable experimental' way. Once he has sufficient backing he could introduce reforms gradually.
But no, he's stupid, shortsighted, and so wrapped up in his own insecurities and inferiority complex that he can't accomplish anything he sets out to do. The worst thing about Al isn't that he's a jerk or a bad guy or anything like that, it's that he's bloody
incompetent. He doesn't deserve the throne and would've honestly been better off if he ran away or something. Hell, he would've been a better villain if he
actually hated his sister, but no. We get...this.
So, maybe it's bad writing or maybe it's just the need for an antagonist and he drew the short straw and they made it stick (which is also bad writing, now that I think about it) but whatever it is, folks unironically cheering for him and calling him 'based' just have a complete lack of understanding of how any of this works, and are just latching onto the only male character with any real presence in the series.