John, you can do a lot of good things in that corner. First off, you need to determine the size of the HDTV that is going there. This will set the width of the unit, and second it will set the debth from the corner. My Tv is 44" wide and 31" tall. On height, for LCD TVs try to keep the verticle viewing angle as low as possible to see the best color.
Find and measurer all the electronic equipment that you want to put into the new unit. Add three inches debth to the biggest one for cables and make sure the stand will be that deep. Design in a shelf for each unit. Minimum would be a DVD. Probably you will have a cable box and maybe an audio receiver too. A simple audio unit would be stereo and you might be able to build in the 2 speakers on the edge of the unit. Doors!!!! All electronics units use remote controls. When ever you use the TV, the doors must open for the remote. How important are doors.
cables: drill 2" holes in each shelf and on the top for the passage of cables.
What if you have a suround sound system. In that case you need to make room for a L speaker, a R speaker, a center channel, and a subwoofer. Where to put them all. Center channel can be built into the top of the unit just below the TV. TV ends up on a flat shelf on top. A covering around it and doors in front of it make a much bigger project. Keep in mind that HDTVs are wide. Not productive to build a cabinate for and old analog TV. They're not making them anymore and in 3 years what you buy as a replacement will not fit. (my 32" HDTV fits in a corner cubbord)
Subwoofer. I built one subwoofer that fit in a corner like that behind the TV unit, and the sound projected from a port in the top. Mine was 15" sq and made the TV stand sit out in the room a lot more. Sub worked well there, but no where else.
Might not matter wheather you build a triangle and fill the corner or not. It will take more wood that way. Think about allowing access( just a hole or removable panel) to the back so that you can easily change a cable or add a unit.
How does the TV or antenna cable run and where is AC power. A surge protector is a GOOD thing, allow room for it, provide a cable path to it. and be sure you can reach it from the front. My $.05 Bob