TUI have slowly built up the BOH base with next summer seeing at least 3 extra flights using 3rd party carriers and a W pattern from Cardiff
I think its always been on the cards for a while that a 2nd aircraft 737 800 possibly could do a small amount of extra work in the summer and share it with Cardiff or Bristol
Regarding the 787 I doubt it , Bournemouth does incredibly well with its fly cruises to Barbados with P&O and Hays travel and regularly go out full , but they have a good year or so sell those seats . This year there are about 5 flights over the season
Who knows , would be great to be proved wrong !!