So many different ways to tackle this, I agree you can't think what might fit her yet.
Wider seat - carries a bigger bottom better but not all horses can carry a wider seat
Flatter seat - more space for bigger bottom and can be helpful for longer legs, and a flatter tree is nearly always better for these shapes of backs anyway. Their backs are flat, curvy trees are not for slightly scoopy backs, they're for higher withers, and have the wrong shape of rails for natives, cobs etc. There is even research saying that narrow twists (wrong shape of rails) are bad on whatever shape of horse, it's all relative, the wider the better generally.
More forwards flap - for longer legs, may still affect the fit as it lengthens the overall footprint even if it is the same seat size and panel length out from under the cantle
Shorter panel - a lot of bluster out there about this. There is ALWAYS a compromise. If your horse doesn't need a deep rear gusset to balance the saddle then your options open up, if your horse is very croup high or scoopy backed, then you can end up with an inch shorter. What is not true is that you can simply order a saddle with a shorter panel. The SMS advises you should only go 1/2" shorter, and we have shortened our panels over the years but it has meant a very slight reduction in the depth of the gusset on some models. For most natives and cobs this works well. If you get it wrong the saddle will tip back, OR you have to widen the tree too much to lower the front and end up with rock/roll and/or the back lifting, OR the flocking ends up all over the shop trying to balance it, ie rock hard at the back and not enough at the front.
So there are solutions, and treeless could be one, but it's not a simple fix - some will slip, many don't spread weight well, and some horses and riders really dislike them.
Find a specialist in wider horses and natives and work with them
This is why I hate saddle shopping and fitting (we also greatly lack fitters around here), its so...complicated to me. The tree with a bit more curve was intended to work for my high wither horse, but still just seems odd on his flat back. But a flat panel could also bridge on a back with more curve, so wouldn't it need a curvier tree?
With the approval of a SMS fitter, who I have to call over from another country, my horse was fit with a saddle that is just a smidge long. He's been fine with it. I do think some brands go nuts with the "you can have a 17.5" seat on a 16" panel" and when I foolishly tried that route years ago it ended with an unhappy horse.