There's probably ten thousand things that you can do with a telephone which can be offered to a business as straight-up monthly SaaS, and even more ways to embed a phone call/SMS into a business/product.
Seriously, if you lack for ideas, pick a business and enumerate all the calls they are likely to make. Anything where you use the common pattern of "Speak to an employee, as an alternative to a REST interface" is likely amenable to Twilio-ization.
Example: I used to work for a large office supply store. The most common phone call was "I ordered something from you on X. When does it arrive?" You know what I'm going to do, right? Ask you for enough identifying information, find your order in the computer, and then read you the expected delivery date.
The most common outgoing call was "Thanks for your fax order. We're calling to confirm receipt, tell you that you didn't calculate the total correctly and the actual price is $42.06 or $3 less than you expected, and inform you to expect delivery tomorrow." This phone call routinely took 20% of order entry time. Everything in that phone call just used me as a voice interface to our database.
A Twilio app would have obsoleted an entire floor of employees at that company.
This kind of anecdote makes me wish it was feasible to shadow a BigCo in a random industry for a week and just spend the entire time taking notes on choke points and practices.
It might be, you could ask. People are also generally quite willing to talk about their jobs, what they do all day, the annoying parts of said jobs. Or you could get a job for a week or a month in a place like that. Actually doing it beats seeing it done for insight.
When's the last time you had to call someone on the phone to get something done that lots of people routinely have to get done? Turn that into a computer program.
Seriously, if you lack for ideas, pick a business and enumerate all the calls they are likely to make. Anything where you use the common pattern of "Speak to an employee, as an alternative to a REST interface" is likely amenable to Twilio-ization.