Create a Twitch Wallet to buy bits or sub
Just as it works on other platforms, I think it would be interesting to have a wallet system on Twitch just as it does on the "bits."
That would lack the form of giving subscriber and would also create a "culture" of adding a certain amount of money per month in the wallet to give sub or buy "bits."
This suggestion would be feasible for people who do not have credit card and accurately go to the bank every month or every 3-6 months to renew only one sub while he could deposit an "X" value and spend as he pleased.
Sorry for English, this is not my native language.

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FXPeanut commented
Yes, many of us small streamers have collected balances below the payout threshold. That money is basically useless and sits for months (or even years) and can't be used.
If there was a way to use that payout balance to buy subs/bits it would allow smaller streamers to benefit even from just a few subs/bits spent on their stream and encourage them to sub to more streams.
The price of a sub would still be the same at purchase (so it wouldn't impact the price) and twitch still get's their split, so it wouldn't change anything from twitch side. The only difference is you could spend your balance instead of paying from paypal/card.
This would work the same as any other service with an available balance... steam, patreon, etc. If you have a balance on your account, you can use it to pay for the things you want.