Adding flights to an existing route (say, going from 2 flights a day to 3, or 4 flights a week to 6) is certainly possible. It's also possible for airlines to add completely new routes. However, these changes are announced months in advance, as any one route change requires changes in other routes (sometimes quite complex, particularly at busy airports where adding a flight means taking away another flight).
It's also possible that routes will be eliminated or frequency will be reduced. Again, this usually happens with advance notice of months.
For next summer, most flight schedules are probably already determined. So, what you're seeing now is probably what you're going to have for next summer.
For Maui to Paris, you should be able to go in one stop using Delta/Air France (via Los Angeles or Seattle) or United (via San Francisco or Chicago). For Vienna, you may be able to go in one stop using United/Austrian Air via Los Angeles. If you want or need to use OneWorld airlines like British Air or American, you're going to have two changes on these routes, as you say.
If you're not locked into any particular airline, use Google Flights or Kayak to see all your options.
To learn who flies where from a particular airport (which is how I came up with the one stop routes above), look at that airport's Wikipedia page.
However, be aware that at any time, a flight that isn't full can be canceled and combined with another flight on the same route. Hypothetical example: if there are four flights a day from Salt Lake City to Oakland on an airline, and they aren't filling, one of the four can be canceled, and passengers re-booked on one of the other three that remain. When this happens, all you can do is ask to be changed to the one still going that suits you best (you can't take a flight that no longer exists). These cancellations usually happen a few days to a few weeks before the flight, so it's important to keep checking after you've booked.
