1. you can mine another Ethash algorithm like ETC
2. can use NVIDIA 1070ti or 1080ti.
whether you're serious about purchasing a GPU for now, I think it's not the right time.
For you the best alternative is Ethereum Clasic.
These cards are expensive 1080ti is 250% of RX 580 and 1070ti is 170% of RX 580.
So is it better to buy one another RX 580 for now?