...I think a jacket and a fabric tarp would cover most situations.

One of the new, lightweight tars might fit the bill

for 20 years i've carried a Land-Shark as my emergency bivi bag

https://www.land-shark.com/how-it-works#features

their description:

"The Land Shark is constructed of a micro-thin layer of aluminized film laminated to a resilient, composite reinforced, ripstop material. The aluminized coating reflects up to 80% of radiated body heat. The tightly-woven ripstop reinforcing seam construction resists tears and punctures, even from boots and sticks. The Land Shark is completely windproof, waterproof and acts as a vapor barrier. The Land Shark Instant Survival Shelter measures only 6” x 8” x 1” and weighs a mere 22 oz. (0.623 kg.). When needed, it opens to a windproof, waterproof, heat-retaining and roomy 38” x 80” (96.52 cm x 203.2 cm)."

A simple poncho and cord would suffice

WalMart has a cheap light 5x7 tarp

A light weight bivvy sack would be very handy for an impromptu overnighter. Something like:

https://www.surviveoutdoorslonger.com/shelters/survive-outdoors-longer-emergency-bivvy.html

This same company has additional bivys, somewhat warmer and more expensive, and others produce similar products. In mild conditions, with a bit of extra clothing, this is all you need.

I think a good rain coat would suffice