Glenbrew Stout kit improve?

You could strengthen it with some dark or extra dark extract. If you add extract, it makes sense to add some hops as well. Not too much, though. Maybe 3/4 to 1/2 ounce of Fuggles pellets at the start of the boil for bittering for every 3lbs of extract you add. How old is the kit you are using? Hop bitterness can diminish over a period of months, so if it is fairly old, you might consider adding a another ounce of Fuggles (or similar).

For yeast, I'd go with a good liquid yeast, either an Irish variety or an English ale variety.

Good luck.

Thanks for the suggestions. I know this "all in one" kit is sort of cheating, but I'll try to make the best of it...