I like TfL's idea of discounts for Tube-bus or bus-Tube through
journeys, but it may be difficult to implement. There would have to be a
limit on the time allowed for a bus journey to a Tube station, and a
limit on the time allowed to catch a bus after getting off the Tube.

I don't see that as being too difficult, provided you don't try to be too
clever about it and try too hard to limit it to genuine bus-tube or tube-bus
through journeys. Just make it a general case that any tube ticket bought
within a certain time of any bus ticket attracts a discount, and vice versa,
where that time could be extremely generous (say a couple of hours). There
will be those who are actually making two completely separate journeys, or a
return journey, who also get a bit of a discount but I don't see any problem
there at all. Just means a lot more happier people surely?

Indeed you might not need a time limit at all. If you were just interested
in encouraging peak time bus-tube through journeys then make it a peak-time
incentive. If you want to encourage it all day then make it all day. Don't
worry about how long apart the actual journeys are.