So here's the thing: you can make this vegan if you use shortening instead of butter. And you can use pretty much any fruit you want, whatever's fresh. If it's a little sweeter, you'll go easy on the sugar in the filling and use a little more lemon juice; if it's tart, the other way.
What you want is to end up with 4-6 cups of fruit. I've used apples, pears, rhubarb and strawberries, an assortment of berries. Last week I made one with peaches and cherries, and that was just lovely. So go with what's fresh and seems like it'd be good together. Cut it up into pieces you wouldn't mind biting into. Slices of apple, chunks of strawberry, whatever.
Add to that:
2 T sugar
2 t cinnamon
2 t lemon juice.
Toss together in a large bowl. Put in a greased 9"x13" baking pan.
1 3/4 c. flour
1 1/4 c. oats
2 1/4 c. brown sugar
1/2 c. nuts -- these can be left out if you're allergic or varied to go with whatever fruit you've picked -- walnuts, pecans, hazelnuts, whatever
1/8 t. each of allspice, cinnamon, and nutmeg
Use a pair of knives, a fork, or a pastry blender (my favorite) to blend in 1 c. cold butter, oleomarge, or shortening of other kind. Alternately, grate the butter/etc. in with the large holes in a cheese grater. Mix thoroughly until it's crumbly. Sprinkle over the fruit. Bake at 350 F for 45-60 minutes (the filling is bubbly and wonderful).
Vanilla ice cream or caramel ice cream on a hot piece of crisp really could be worse.