When it comes to features:

• it has to be a HD wallet: i don't want to make backups of my wallet file every couple of days
• it has to be encrypted
• native segwit
• coin control features: i have to be able to pick the unspent outputs i want to use as an input
• tx control features: i want to define the change address, or let the wallet derive a change address for me, i want full control over the fee, i want to be able to create unsigned transactions or sign unsigned transaction i constructed outside of the wallet
• i want to be able to sign messages
• the wallet has to be open source, signature has to be available
• i have to be able to build the wallet on *nix

There are more features, but i probably wouldn't even want to use a wallet that doesn't offer above features as a bare minimum.

The UX, i don't care about... I use core from the command line, and electrum aswell... Sure, a gui is nice sometimes, and if you want to create a wallet that's newbie-friendly you really need one... But if you create a wallet for older users, i wouldn't worry about the ux all that much (at least, that's my personal feeling, i know a lot of old-timers that DO care about UI/UX)