For the love of god, if you have a mutualy beneficial deal with an AI (luxury they don't own for luxury you don't own, alliance) and the deal expires while the AI is still on good terms with you and would agree to prolong to deal, make the AI offer the deal to you!
While we're on the topic, make it so you can offer alliance and it automaticaly counts as friendship declaration instead of it being separate actions.
Mentioned several times, but teach the AI to use air units! It's such an important part of modern warfare, yet it plays little role in Civ modern time battles.
Again not the first person to suggest this, but do some systemic buff to tall/nerf to wide! I think easy solution would be making specialists better (add GPP, additional specialists yields locked behing certain technologies - possibly something like "specialists working in districts with powered up building +1yield").
Make politics concerning city states more interesting! Add pledges of protections, add ability to demand from an AI empire to cease attacking a CS, make your allies who wish to stay your allies not attack CS you are suzerain of.
Yeah, to me every "XPT" for "XPT" type deals (ie. lux for gpt, or lux for lux, or alliances) should have a simple auto-renew functionality if the AI is still willing to continue, and ideally should be able to be cancelled anytime. ie. if I'm trading cocoa for 7gpt, it should continue for at least 30 turns,so either side could cancel after 35 or 40 turns if you think you can get a better offer, but should be handled automatically. Most of the time I only find our one of my deals is finished when an AI offers me garbage for a luxury it reminds me "hey, wasn't I dealing this to Mali previously?"