So much depends on your taste, also if you intend to put lights on it.
Lots of real ivy, and greenery - outdoor Christmas tree sellers have lots of chopped off branches - (some will sell, some will gift them) that you can weave in. If you can afford or beg a few eucalyptus sprays, the combination of the three natural greens really works. If you want lights, small red ones in clumps of two and three looks tasteful.
You can make your own very effective red berry groups with Q tip ends, wire, and £1shop red nail varnish if you have time, and pinecones are free and can be left natural, very lightly sprayed metallic, or entirely sprayed - again £1shop for cheapest spray paint.
If you are putting cold lights especially blue on it, then spraying your natural materials white and silver, and staying with white silver, and clear ornaments also works.
The trick with shatterproof baubles as fill is is to have some mat and some gloss, if possible in two slightly different tones of the same color.
Depending on your taste ribbon can make a good filler especially florists ribbon which can be curled. With all ideas consider what things will look like wet, and avoid things like walnuts, acorns, real berries, etc, as wildlife will pull everything apart and risk injury trying to feed.
Ikea shatterproof are a nice quality and can be picked up 2nd hand on Ebay (there's currently 50 gold ones very reasonably priced) or in October their red ones work out at 25p each -20 for £5.
B&Q's say indoor use but it depends how sheltered or exposed your arch will be- they have 40 silver, gold, champagne, red, or red green mix for a tenner.