I have some space and some high trees. I already use a 400ft doublet with
atu for several bands and a 30m half square and get good results. I want to
improve my performance on 17m. I am now thinking about a wire antenna with
reasonable performance for 17m only. I have thought about a full wave loop,
even a bobtail. A broadside array would be great but I am not keen on
antennas with reflectors or directors because of the complexity of support
ropes. So, really some kind of antenna supported by ropes from two trees
would be ideal. Any ideas please?

As others have said, you need to decide where you want to point it first,
because something with significant gain is going to be directional.
For 17m you probably want to go horizontally polarized if you have high
trees.

You might consider a horizontal vee beam, the lengths and enclosed angled
are relatively non-critical. It would also work well on 20 and 15m. I have
also constructed tree-supported yagis (wire) and quads, but you do need to
tune the parasitic elements carefully to get good performance.

Two triangular quad elements (flat side up, fed at bottom point)
separated by three PVC pipe spacers makes a nice quad that will hang
between only two supports. I used to have a couple of those up for 20m and 15m.

Tor
N4OGW