you have no driving game. Get one of those. Or a shooter. That would be a good addition. Something like cheyenne, crossbow or chiller.
The drivers that I like the most are Sprint 2 and (fast-forward!) SF Rush. Sprint 2 is pretty big I think, and SF Rush is REALLY big, and you really need two of them at least to get the most out of them.

Can you think of a great driver that is standup? I don't think Pole Position would last very long for me.
I have got to concur on the Neogeo system, but I'd go with a multi-cart instead of relying on the multi-slots. Excellent 2-player games- and the best the 16bit gen has to offer...
There aren't too many Neo Geo games that I'm really into. I'd love to get a single slotter with a multicart or even just Puzzle Bobble to play with my wife in the Double Dragon cab. I find that games like Metal Slug don't work very well for me when you don't actually have to insert coins. There's no downside to dying. I don't raelly care for fighters too much either.
Discs of Tron, Star Wars, 720, Paperboy, Multi Exidy Collexn'.
Out of these, Star Wars is the most interesting, though I think I like Star Castle more.
What about a pin? Every gameroom needs a pin! Earthshaker, Taxi, Diner or Whirlwind.
If I were to get a pin, it wouldn't fit in the space that I have left. Also, my wife is way more into pins, so she'd have to put it in her hobby room!
you could always make a custom panel and play neo geo in drouble dragon cabinet... Neo geo pcb with 150 cart would be a good choice.
If you have not done it yet, drop multi williams into the defender cab. If not, get rid of defender and replace it with a jamma cabinet.
If your going to add 1 more cab, I would say a SF setup cab. Maybe something you can rotate the monitor in easily so you an play horizontal and veritcal boards.
I don't think I need more than three buttons for the neo geo games that I care about. The Double Dragon is easily rotatable to vertical/horizontal, so that's covered pretty well.
The Defender usually has a JROK board in it for Defender and Stargate (minus Inviso). I'll make a panal for it at some point with the extra button (don't want to hack up my Defender panel!) The only reason I run the original Defender hardware now over the JROK is that I haven't worked out how to get the same frequency response on the audio yet out of the JROK board.
I don't like fighters much, and anything with more than three buttons is too much for my tiny little brain.

If I wanted a 6-button layout, I could make another panel for the DD. I already made a 6-button layout/2-player panel for the Defender when I first got into this stuff and thought I'd be able to cope with Mame, but it went back to being a Defender.
I'd suggest selling the double dragon cab and replacing it with a japanese candy cab such as an egret2 or sega new astrocity to serve as your jamma/non jamma pcb workhorse cab. You could get one with a 2 Player 4 button neo style control panel and have a spare 2 player 6 button control panel for SF style games. Then grab a double dragon pcb and slap it in there. This suggestion is if you are considering playing neo geo games. That way you wouldnt have a homebrew looking custom cps, which tend to look kinda tacky imo.
Then I'd look to add either a 2nd candy cab so you could have one set up for vert and horizontal games or Spy Hunter or Front line.
Tom
I don't typically like cabs with bigger than 19" monitors. I'm pretty happy with the DD as a jamma cab, and it's easy to rotate the monitor. When I make panels, they won't look tacky.
Made Planets is one that I might consider over Star Castle. I'd also love to have Q*Bert. If JROK finishes his Q*Bert/Made Planets, etc, fpga board, I'd love that and build control panels for Q*Bert and Mad Planets. From what I can tell, getting those to work with a jamma adapter is a pain in the butt. I'm hoping that he finishes that project!
Monaco GP is also interesting, but it sounds like a nightmare to get one of those together that works and keep it working.
I'd get a Space Duel myself.
I've never been into Space Duel much. I like the idea of co-op Asteroids, but it just doesn't seem to have the soul of Asteroids, ya know?
Someone mentioned a dedicated Mario Bros. That plays really well on the DD with a jamma adapter, so that's covered pretty well.