I don't think it's essential to hit all three criteria: although 1 and 2 are essential for it to be a quick win, there are ways around 3 - but the risk of not hitting it (or something like it) is that there will be complaints of the 'why are they getting something when we're not?' type. Against that, there's the risk that failing to do anything will lead to negative press: the current out of use fleets are an example - we can assume that the Daily Mail will have a field day when they decide to run stories on those.
So my suggestion would be to work out what is inevitably (or near inevitably) going to happen when GBR finally comes in - and then see what of that can be brought forward to (positively) happen now. On that basis, I'd suggest
- unused fleets: I'll accept that this is something I haven't been following so I may have got the wrong end of the stick here. But if there are fleets of modern trains sitting unused with TOCs or ROSCOs then get them lined up for use, ideally as fleets so that crew can work them consistently. If the trains are with the ROSCOs then someone will have to rent them - but if GBR will be the sole operator in future then the government is in a good position to insist on the ROSCOs offering a good price - there's also good politics in the new government being firm with greedy private ROSCOs. And crews will have to be trained: again, it might be good politics to show that if there will only be one operator in future, then the unions will not be in a position to play employers off against each other for double crewing (driver and guard) etc - it's perhaps worth noting that RMT is not affiliated to the Labour Party, although ASLEF and TSSA are.
- single TOC fares: as I understand it, in effect all the farebox from all TOCs (open access operators excepted) now goes to the DfT. So there's no benefit to TOC in gathering a bigger chunk of the fare-take on a given route. So let's abolish fares divided by TOC (e.g. 'Northern Rail only'). Commuters from Liverpool to Manchester (and similar routes) won't have to worry about whether they're on the right colour of train - more choice if train for very little more money. But let's keep Advances: to also abolish cheap fixed-train journeys would not go down well (Daily Mail: LABOUR PUT UP THE COST OF MY COMMUTE BY ALMOST 200%*) and (for sensible lengths of journeys) Advances can play a real role in moving traffic on to more lightly used services.
*Per BR Fares LPY-MCO (and vice versa -
https://www.brfares.com/!fares?orig=MCO&dest=LPY), comparing cheapest Northern Advance x 2 against off-peak return - my actual commute (using the off-peak fare rather than the Advances)