I think the Z4 (G29) would be perfect for an Alpina version. After all the Z4 (with possibly the exception of the E85/86) is not a sports car; it is a 2 seater convertible grand tourer.
In recent years they have offered a slightly left field alternative to BMW's 'M' cars. As BM won't offer a Z4M G29, then surely an enhanced Alpina version would be pretty special. One thing that Alpina are good at is offering a 'base' car, in this case it would be the M40i, but so much more. albeit at so much more money.
If they have let AC Scnitzer take up the slack, it seems a shame IMHO, as an Alp is far more than a remap and bodykit.