Deliberately open-ended question - this can be in terms of rolling stock, timetabling, branding, station facilities, or anything really! Including both current and past TOCs.
The experimentation also doesn't have to be positive - it can have been detrimental, or simply have gone absolutely nowhere...
Has to be Virgin. Whatever you might think of them, for innovation nobody came close - and the end result was a pretty reliable TOC, too. Avanti have truly ruined it in a very short space of time, just like many of us thought they would.
(of course we mustn't forget their abject failure - XC - which has suffered from their poor choices ever since - so not all experiments work!)
Great idea for a thread.
Looking at the OP's definitions, I'd argue that Cross Country has a good shout for being the second highest in a list of "innovations" (West Coast being the clear winner) - they inherited the dregs of BR's "InterCity" operation and replaced them with a fully new fleet, turning a mess of services into a clock face timetable - it's easy to forget now how ambitious the first version of "Princess" was - bi-hourly to Dundee/ Cardiff etc - before being scaled back to something more manageable.
I suppose one difference is that the Government allowed West Coast to order additional trains and extend existing ones, whilst the Cross Country franchise was never given the same permission to expand their fleet - e.g. when TPE took over the Manchester - Scotland bit of VTWC the Voyagers freed up went to beed up the Chester - London service rather than going to XC where they were more needed.
I'm not saying it all worked, clearly not, but the difference is that some TOCs got a second chance to get things right once their first attempts were translated into passenger numbers - but XC didn't have the Government backing required to allow them to build on the boosted passenger numbers that they were struggling to accommodate.
Hard to know with some other TOCs how much of it was due to the minimum requirements in the franchise documents and how much of it was due to genuine TOC innovation (and I think that sometimes we can over-egg the benefits of a once-a-day extension of a London service to put some provincial town on the map) but credit to Midland Mainline for doubling the number of services from London to the East Midlands (every forty five minutes thirty years ago, every half hour at privatisation, four trains per hour under National Express's franchise).