There's not a whole lot to explain here: I think it would be great if there were more 'boss' monsters. Currently we've just got the Tortodrone and the Trojan (who's more of a miniboss, really). There's also that fiend lord, but no one's been able to fight him in forever.
It'd be really cool to have more enemies like the Tortodrone: solitary monsters with huge hp and multiple powerful attacks. Fighting multiple waves of gremlins is alright, but every time I face off with a Tortodrone I have a memorable fight.
I think the problem is the lack of variety in the gates at the moment. I can't say for certain, but I'm pretty sure once we get more gate variety we'll get more bosses.