A barebones plan
A 'basic' price plan between 'free' and 'plus', that is a lot cheaper than the $/€4 per month 'plus' plan. The price covers the running costs of a 'free' account, and should provide less of a shock to the system of cash-strapped people used to free (albeit not private) email.
The added features over the 'free' plan would be Protonmail Bridge, so emails & attachments can be downloaded, as the 500MB capacity fills very fast even with just standard usage, and the option to remove the signature "Sent with ProtonMail Secure Email" (default setting would be signature included). I would personally be happy to keep the signature to help spread the word about Protonmail.

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RejZoR commented
I agree. "Basic" plan should also exist. Same as "Plus", but with only 2 custom addresses and no custom domain. I'd take that without hesitation if it was 214€ a year, but for 48€ a year and not having such basic capabilities as browser extension notifier, I just can't decide for it. It's very unfortunate because their mission of secure e-mail is on point. But it just lacks so much and it's quite expensive if you don't really need all those extras. I really just need more space and folders/labels than Free plan offers it.
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M commented
A 'basic' plan would be a comfortable and accessible first step, which could attract more 'plus' and higher members.
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M commented
Perhaps also have a ProtonMail desktop app that incorporates Protonmail Bridge, so you pay your $/€1 per month or whatever it is, and you only have to have one login and password. I know many technological luddites who would welcome this simple setup.
If making your own desktop app doesn't grab you, you could ask Mozilla for permission to make a ProtonMail app 'Powered by Mozilla Thunderbird'. The program would look similar to the website version, with all the features like seeing storage status, but with some extra features, like a 'transfer email to PC' button, and have it clear if you have the email on your PC or on the ProtonMail server, or both.