My choice would probably be a jacket too but to my own design. Failing that, If it was 10 or 12 oz then I would make a rain hat or two (Aussi style) & a rain coat for my ageing hound. Bags tend to be heavy but I've always had a hankering for a set of saddle bags slung over my shoulder in stead of a day sack so they would be on the drawing board too. Waterproof outer mitts, gaiters, a pair of trousers, pouches,map cases etc, could also be on the list. Don't throw away any scraps either as folks are always looking for bits to repair their jackets & coats.
A poncho may be OK, Driza-bone make one but it's not the type of thing you could just sling in your rucky in case it rains, way too heavy & bulky. Probably too hot in summer but fine for the rest of the year & if you make it so it can open like a cape (buttoned or poppered front for example) then you could control the temperature.