There’s no way to extend the algorithms unless Steinberg may add more.
Anyway, the Elastique Pro algos work great for me on 95 % of all material. There are some cases that tend to produce artefacts (warping sustaining pianos is weird sometimes) but most stuff gives me ‘natural’ results. Can’t compare it to algo from other DAWs as I don’t use others. I can switch to Melodyne if nothing ‘onboard’ pleases my ears.
I’m really curious what kind of audio material Cubase failed to time-stretch, while Logic was successful. Have you tried every algorithm in Cubase? To me, MPEX algorithm which comes with Cubase (among several others) is the best I’ve ever encountered.