Pretty much zero. Plans can change of course, but with the 95 stock now being past half life (assuming 40 years) it just wouldn’t be worthwhile buying either extra trains to form a mixed fleet, or 95 stock clones. The latter probably wouldn’t be a viable option anyway.
The only remote possibility is if the Jubilee fleet were to be replaced, it might be relatively easy to convert some of those to run on the Northern. However there is enough difficulty replacing the 50-year-old Bakerloo trains, as well as the ongoing debacle with the 92 stock, so the prospect of anything happening on the Jubilee is as remote as ever. Extra trains for the Northern would also require extra stabling space, though there is a bit of spare land at both Highgate and Edgware, and potentially subject to some minor work three trains could be stabled at Golders Green in the sidings south of the station, and there are three siding unused roads at High Barnet. Space for 10-12 trains should be fairly easy to provide. Beyond that will require more significant work - and it won’t have helped that Khan is busy trying to use car parks for housing schemes, as that’s more space gone!
Not ideal. As in addition to going from 20tph to 25tph, you’d be introducing the problem of trains having to detrain at Tooting Broadway, and the siding berth there is so tight that the entry is very slow. Even if it were theoretically possible to timetable this, it would be introducing a significant performance bottleneck, which might well ultimately pull down the entire line. Also worth adding that by not reaching Morden, the extra trains would be of direct benefit to fewer people, which further reduces any case for them.
Another problem with making regular use of Tooting Broadway is that there are no facilities for drivers there. This isn’t ideal in itself, and would likely attract some resistance from the unions - and probably reasonably so.