We have a room like this. At the same time as we had our extension/new kitchen, our neighbours did the same thing, only on an even bigger scale (both did it last summer). Both really pleased with the results.
We added a small extension on to our strangely shaped kitchen (which was wide/square when you walked into the room, but narrowed down into a galley kitchen. We turned it into a big, rectangular kitchen and dining area/breakfast room). We also replaced a fugly old side door with french doors at the end of the room and to one side, leading on to the garden. We chose to add some new, larger windows to replace some small, boxy windows, too. Our neighbours chose to put in a skylight. Skylight has more of a wow factor (and cost more, too!), but either way, both options let in more light, and you have to consider this seriously. The bigger the room, the more light you need to let in.
I would say - keep it simple. Our architect came up with three different plans of varying degrees of complexity and we chose the simplest one.
Also, think about the big basic. How are you going to keep a larger space warm? Will you splash out on underfloor heating? Or put in a new radiator or two? If you do choose some sort of skylight/conservatory-style extension, how will you keep it cool? Will you need special blinds made, fro example? What type of flooring will you have? Real wood will be very expensive for a large space, but might be cosier underfoot than stone or slate. You will really want something easy to keep clean, though!
Waffling now, but am happy to answer any other questions you might have [
