Agree with Bob, it is not an exact definition, just a nick-name which came into use in UK when people started seeing small round cars with relatively big bubbly windows, principally the early "bubble window" Isettas, 'shmitts and H/Ts. The name was already known before the Trident came out but it fits the bill, as does the later Isetta: despite having less bubbly windows it still follows the bubble shape. BMW 600 and original Fiat Multipla have flat fronts which could just see them included too.
Others could include Arzen's Oeuf, the Hoffman "copy" of the Isetta, a few Eastern European, Spanish and South American cars like the Dinarg, and arguably the Delta/Janus.
As Bob says, most of these are well known, some in bubble car books, some in micro car books, some in Eastern European car books (like Julian Nowill's) or other foreign/classic car books.
Just limiting your future to just "proper" Bubble Cars you have a very limited number of proper contenders, and therefore you would have to have something new or interesting to say about them, and/or get some really interesting photos and locations. The photos in the Sparrow Family Albums are hard to better, so that might a visual bench-mark to aim for. If you go more on text you may frighten off casual purchasers but also struggle to get many sales among proper devotees as a lot of info is already out there in books, magazines and club publications.
That's the bad news, the good news is that most of us on this site are compulsive anoraks and never tire of seeing more published! But certainly try to think of a "new" angle, good luck and keep us posted!