I’ve heard great things about Gravit Designer (https://www.designer.io). It apparently is close to achieving feature parity with sketch. I don’t think Gravit supports plugins.

Alternatively, you could use InVision Studio (not released yet) or even Figma, but those are web-based, so maybe not such a great idea.

It depends on what you're doing, but I'm here to tell you that unless you're doing Visual Design, you can live without Sketch altogether.

Where I'm employed at the moment we use Sketch for Visual Design. For pretty much all stages before that, we use Google Slides - this includes everything, from information architecture, through UX, sometimes right up until the point where we really should be asking some hard Visual Design questions.

If your primary concern is UX or prototyping - you can do it with pen and paper just as well. To be honest, I generally find that it's best to start lo-fi. Sketch invites you to try to make your wireframes pixel perfect, which you probably don't want. Sketch also entices you to start using symbols which can lock down your designs too early. Slides will not draw you into selecting fonts, or dealing with exactly what color you want to use.

So, if your goal is to do wireframing, my tip is to go lo-fi.

However, for VD, I'm not sure you can do better than Sketch at the moment... It is a sad reality. I would have seriously considered ditching macOS for a GNU/Linux alternative if it had been possible for me.

Figma is currently the best solution for UX design needs. It works quite smoothly in the browser. They've taken advantage of asm.js to the fullest.