Is it possible to withdraw other cryptocurrencies besides bitcoin (ex: etherum) into a Bitcoin paper wallet? I've been wanting to know if withdrawing other cryptocurrencies into a bitcoin paper wallet would be incompatible with that system.  I haven't tried it myself.
It may work, depending on the currency.

You will likely be able to send BCH to a BTC paper wallet, and LTC to a BTC paper wallet with a 3-address, due to address format overlaps.

You will not be able to send currencies with other address formats, such as Ethereum, Monero, ZCash, etc. 

Do note that even if you do succeed in sending the currency, there is no guarantee that you will be able to spend it. For example, sending BCH to a BTC segwit address will work, but spending it will be extremely hard, as segwit is not enabled on the BCH network.