I suppose it depends on the aesthetic you're going for, but in my opinion, you can't really go wrong with classic tattoo imagery...roses, birds, skulls, flowers, dragons, animal heads, daggers, lady heads, nautical and maritime designs, or any of the above in combination. I know from Instagram that the tattooers at Frith Street are always drawing and painting flash (lately I've been really digging what LM Knight has been doing) and I'm sure they have sheets and books of possible designs that you can look through and choose from if you go there. For me, a big part of the fun of getting a tattoo spontaneously is picking something that feels right at the moment and just going for it.