While the online gambling industry is huge, I have yet to see any platforms that offer services in the informal gambling sector.
The concept would be simple, such a service provider would simply act as an intermediary for two parties wagering money on the outcome of something. The legality of such a service would depend on the local gambling laws, these laws differ so much from country to country or even state to state that a lot of research would have to be done before offering the service in your area. (I do, however, believe that since the platform would not be acting as a casino or bookie, there might be some loopholes, because the company would effectively only be operating as a service to send and receive money.)
In operation, the service would simply have two parties deposit their respective bets (because this is for betting against one or more parties, rather than against the house, there would be no need for odds, betting markets or multipliers) and keep the funds in escrow until both parties have confirmed who won at which point that party would receive the winnings minus a service fee.
In cases where the outcome is disputed, the company can investigate (for an added charge) and decide whether someone has won or the bet is voided in which case everyone receives a portion of their deposit back.
This service would be great for people who want to make a bet amongst friends or bet on obscure outcomes not covered by betmakers.
Depending on the gambling laws of a specific region, such a service could also act as a platform for people to make bets with strangers. Users could list miscellaneous challenges that other users could search for. These could be anything that has an outcome that could be wagered on.
For instance, one could take a chance and bet $50 on a coin flip. You would then upload your challenge and as soon as someone accepts it, the platform could have a virtual coin flip for you, or it could allow for a live video during which the winner will be determined. (There might be some security concerns if a physical coin that is in the possession of one of the betting parties is used)
The range of possibilities is endless. You could bet on whether it's going to rain before or after a certain time of day, you could bet on who can stack the most boxes without having them topple over, you could bet on anything, as long as there is a clear outcome and someone betting against you.
Like I mentioned, there are some security questions that would need to be addressed, but in general, looking at the size of the gambling industry, I believe there is still a major gap to be filled for "informal" bets.