Having lived in Walthamstow for the past 27 years, it is interesting to see how the line has changed over the years.
In 1991, the first train from Walthamstow Central was at 05:50, and services were considerably less frequent than today, with a good proportion of the trains turning round at Seven Sisters. On Boxing Day, services only ran every 15 minutes between Brixton and Seven Sisters.
Now you have the first train at 05:24, trains more or less every minute throughout a good proportion of the day, most trains running all the way to Walthamstow, services over the whole line on Boxing Day, and of course the night tube at weekends.
I think Walthamstow Central urgently needs a rebuild though, as the frequency of trains means that there is often serious congestion at the bottom of the escalator, especially during the evening peak. A second entrance leading directly from the shopping centre, or perhaps also from the walkway linking Queens Road to Central, would be useful. This entrance could be made fully accessible.
As to the future, I don't think there is capacity for any extensions, however useful they might be. Perhaps North - South Crossrail, if and when it ever gets built, will take some of the pressure off the line.
One of the Victoria Line's greatest strengths and weaknesses is that it connects with so many other lines.
A strength because every station except Pimlico has at least one other connection to another line, thus improving connectivity.
A weakness because this means that you get a lot of commuters all trying to get on the trains at the same time. If you have ever seen a Southbound main line train stopping at Tottenham Hale in the morning peak, you can easily get 150 people getting off at once, most of whom want to transfer to the Victoria Line.
And while we are on the subject of Tottenham Hale, whoever decided it was a good idea to close the direct entrance from the footbridge to the tube ought to be lined up against a wall and shot.
I will be moving away from Walthamstow soon, so will miss the Victoria Line, although not the overcrowding at peak times.