To anyone trying to achieve the same and who's not in the need to use API v3 at any cost:
There is still an option to get future events with a request similar to API v2. Therefore, use the private XML-code from the calendar settings and add the parameters you need. E.g.:
HTTP GET http://www.google.com/calendar/feeds/your_email/private-validation/basic?futureevents=true&max-results=1&singleevents=true&orderby=starttime&sortorder=ascending
I could not find any up-to-date documentation from Google about this but it's working. The main difference to API v2 is the "/basic?" at the end instead of the old "/full?". Otherwise most parameters seem to work fine.
futureevents=true - only shows future events
max-results=integer - only show integer number of results
singleevents=true - handly reoccuring events as single events
orderby=starttime - sort results by their starting time instead of last update
sortorder=ascending - well, sort order
"printpretty" seems not to be supported anymore. Also, the timestamp has a different look now. But still, all the information needed is included.
The example above shows the request for the next event in the calendar. This may be extended to multiple events (e.g. max-results=5). The other parameters are needed to show up the next events as expected instead of some seemingly random events. The problem with this method is of course that your private adress is used in the request which may be insecure in comparison to API v3.