EGCC_MAN pretty much has it covered.
I wouldn't be the slightest bit surprise to see based ops from Vueling and Sun Air.
Sun Air because they are going down the corporate contract route so a potential based aircraft could cover routes from this end, and be able to run more competently the new Groningen flights as well as another potential new destination to come this winter.
Vueling is quite self explanatory as to why they would base here.
American Airlines and Finnair are joint venture partners to IAG, so, profits are shared between AA/BA/AY/IB, with BA driving a fair bit of growth through those channels.
AA has upgraded both JFK and ORD now, supposedly looking into DFW and will make ORD year round again. Here's a fun fact, more passengers on the AA JFK actually book through the BA code than the AA code. So, it's not as 'London Airways' as some think, as the AA JFK owes a lot to BA.
Finnair will be increasing to the A319 on the morning flight this winter, and IB are supposedly going to increase Madrid but not made it to booking just yet.
I do think it's a crying shame that Citiflyer is based at Manchester, yet doesn't operate a single route from Manchester. Hopefully their STN experiment pays off and we see a replica at MAN, after all, STN is a MAG airport and CFE is a MAN based airline, it's not a silly suggestion.
In terms of BA metal, I suppose the night stopper could be used in the same way they did at EDI last year and GLA this year, and run an overnight flight to somewhere like AGP/ALC/IBZ/PMI/MAH, as those routes seem like a licence to print money at the moment.
So, in conclusion, BA is already very big at Manchester, let's not forget BA mainline themselves contribute over 770,000 passengers a year to Manchester, the issue is, their size is not as evident as direct long haul on their own metal, or, the nice shiney A380 that steals the limelight