What lens should I get next?

There are only 2 Canon lenses in that price range that are worth having.

The EF 50mm f/1.8 - a great portrait lens

The EF-S 55-250 f/4-5.6 IS - a very good short-medium telephoto zoom.

Adding an adapter to an FD 80-200 F/4? It was a very good lens. Adding an adapter will degrade the quality. In a addition you will only have manual focus and stop-down metering. Probably more trouble than it is worth.

It depends on what type of photographs that you want to take, a wide angle lens will let you get more of the scene in shot, at extremes this will give a fish eye effect. macro lenses are best for close ups but aren't versatile, a greater zoom such as a telephoto lens will give you the opportunity to take shots of things which are further away, compress the view, and blur the background, using a low f stop at maximum zoom.

It's also worth considering that fixed focal length lenses are higher quality, as zoom lenses give greater versatility at the expense of quality.

I would recommend checking out a good photography book for examples of shots you can take with different lenses.

Like my wife always says, go wide and fast.