Should this be an external program which gets rendered in a terminal? Maybe...
Change of plans: calcurse is quite nice and should support everything we want. Let's add a contrib script and calcurse config which arranges things nicely and encourage distros to make an aerc-calcurse package that rigs everything up for you. May be nice to send some patches upstream to calcurse which rigs up some stuff to feed events back and forth with aerc to wire them into emails.
Another project of note: DavMail. It's basically a proxy server that converts Exchange protocols into SMTP/IMAP/CalDAV/CardDAV. I've used it successfully to get Thunderbird+Lightning to speak to corporate O365 at a prior job. Packaging wise, aerc + calcurse + calcurse integrations + vim + git-send-email + DavMail could all be dependencies of a singleI need to replace Outlookmetapackage.
There's also vdirsyncer that allows you to sync local event with a caldav instance. They can be then read locally with a tool like khal which has a pretty good interface too, but less shiny than calcurse it seems. Advantage here is that you don't stick to an app that handle everything under the hood (like calcurse do) but rely on just using standards locally (like registering ical files to the local FS). Sounds more like Unix philosophy, dunno if it fits your needs :)