Meal Planning for the Unplannable

So I have tried a few meal planning rotations and love cooking. But I find that since we eat seasonally and closer to our front door it is harder to predict what we will have available in a weeks time or even a couple of days ahead. Maybe we will find a mess of oyster mushrooms or the neighbor drops off a 55 gallon barrel of sweet corn. Now of course preserve to use later is a important tool for spreading out these blessings but when there's a fresh glut I like to base our meals off it. I have a loose system of vague meals that can take a lot of fluctuation of ingredients that I pick from.  Such as Soups and chilies on Saturday, Leftovers on Sunday. Monday we have a meatless bean dinner. Taco Tuesday is more of a general Mexican theme. Wednesday is for rice and a topper. Thursday is not my problem and I make someone else cook. and Friday we grill stuff. This normally works for us but come the hot months it gets busy and I will stop using it.  I want to try changing how we do this and was wondering what others do to help them keep cooking to provide three meals a day, every day, for every mouth at the table. With a pantry of ever changing ingredients.

what a great question.
I also get the glut issue, and I think I address it the same way you do, with "themes" every day. But the themes are very, very broad- one day Turkish with flatbread, one day Japanese/Chinese/Korean with rice or noodles/bread, one day soup.... When I feel like it I'll add in some kind of beans/rice day, Ethiopian day, pancakes with toppings day, etc. It often depends mostly on how I'm feeling.
But I do the "themes" mostly because I hunt online for recipes and it is absolutely overwhelming to search for "recipes with potatoes" or "too many mushrooms", know what I mean? If I can somehow whittle it down a bit ("Chinese mushroom recipes", "mexican potato recipes", etc) it just seems easier to get a handle on things so I can go ahead and whip a full meal together (since now we're really not going out anymore, and we need enough leftovers for lunches too. All our lunches are leftovers, I do not cook lunch, period.)
Recently I've started trying to make a more concerted effort to save the 'keeper' recipes that have become standbys (for example, today is 'Turkish'. which means we'll have whole wheat flatbreads, tabbouleh salad, a bean/soy protein/tomato/pepper stew, and maybe some roasted cauliflower and yogurt sauce). All of these I make so often that I don't need a recipe, but if my daughter wanted to make them I'd like to have it written down-- thinking about what would happen if the beloved sites I originally got them from went off the air (which happens). This  is sort of an ongoing long-term project for me (and an entirely different thread).
But yes, I am ALWAYS searching for what to do with "too much XX".

What dishes can almost anything go into?

chili
curry
other soups and stews
frittata
taco/quesadilla
grain bowl
pasta sauce
salad
stir-fry (and fried rice the next day) or a "skillet" meal
either pickled or served with pickles
baked potatoes

Tereza and Chris have covered most of the solutions, however, I’d like to add roasted tray bakes of a glut of summer vegetables.

Spices can be added at the time of roasting or the vegetables can be refrigerated/frozen and used in other meals - add a dressing and eat as is; make into a ratatouille style dish; added to frittata and/or a quiche.

I also enjoy making pickles, chutneys and relishes so excess produce can be made into a condiment to spice up cold meat, sandwiches, quiches etc

Dehydrating produce is one of my favoured preservation methods and I’m still using dehydrated birch boletes and wine caps foraged several years ago.

I find it helpful to keep some cooked meats in the freezer, in one-meal quantities. I can use this as a base for a hasty meal that I can whip up in half an hour or so with whatever else I have on hand.

- Roast chicken or turkey
- Ground beef, fried with coarsely cut onions and lightly seasoned with only salt and pepper
- Ham (the good stuff, from a big bone-in ham)