After I get my Tandy 1000SX set up, here are some upgrades I'm considering to make it more useful:
I have a CGA to VGA adapter already that was in an arcade cabinet, but which is not in use, but I may need a different one as the board I have doesn't support the intensity line, but, IIRC the CGA standard and, by extension, the Tandy graphics, requires it to get 16 colors. Without it, I believe only 8 colors will display.
Since it has dual 5.25" 360K floppies, I'll likely remove one and replace it with a 3.5" 720K floppy, or, more likely, floppy emulator. I can then put the removed 5.25" floppy in a desktop and give myself a way to create floppy disks.
There is no hard drive, so I will likely add an XT-IDE interface with a Compact Flash adapter. I always hated swapping floppies, so this, or at least the above floppy emulator, is a must for mass storage.
An 8-bit network card. Intel made an 8/16 model that is still reasonably priced on eBay. I could probably get away without the XT-IDE if I had networking, as I could mount a network share.
A Sound Blaster Pro. I like Tandy's built-in sound much better than a standard PC, but, I prefer a sound card. And even though the SBpro is a 16-bit card, plugging it in to an 8-bit slot will still work as long as I don't try to use a DMA channel that's only available via a 16-bit slot. The SBpro also has a header on it that will take a speaker connector for input, allowing me to pipe the Tandy sound through it to external speakers. This gives me the best of both worlds when it comes to sound.
The SX I purchased appears to have a modem and serial card in it, so neither of those should be necessary. Since I would like to add a standard parallel port, I may replace the serial card with either a serial/parallel or a serial/parallel/game card, especially if I don't add the SBpro above. I'm not sure if you can use the joystick port from either without conflicting with the onboard proprietary ports or not, but, if so, I wouldn't hesitate to try. If I forgo the SBpro, if I get a parallel card, I can build a generic version of a Covox Speech Thing. IIRC I had built one back in the day, but can't remember which machine I had it connected to.
Last but not least, I'm considering a VGA card. Obviously it's better graphics that TGA, but I'm a bit torn on this one. Part of the charm of the Tandy 1000 line is the Tandy-specific graphics and sound modes.