Part of me thinks they'd be spending some of the resources combating Briton piracy - but substantially fewer than they were spending holding Britannia. I do think that instead of conquest, we may see one Legion stationed in N.Gaul to assist allies in Britannia and Hibernia, in order to create one stable polity to trade with, rather than dozens of tribes, as having a friendly trade partner is much cheaper than lots of pirates.
After that, excluding Britain, there are 4 avenues of expansion where those resources would go.:-
Carpathia/Dacia,
Germania,
Arabia,
Mesopotamia.
This is the Romans, they did like to expand if they could.
The Eastern half the Empire still retains its dominance economically, and becomes even more important as all the most financially profitable expansion opportunities are nearby, with Germany a poor fourth financially, but in terms of strategic opportunities, a wiser choice for defending the west than conquering Arabia, for example.
If the Empire is divided - then the West will likely invest resources in conquering Germania, or massive army-build development programs (hopefully, maybe, probably not). The East, probably redirects resources that went to subsidizing the West to expanding south and east.
If the Empire is united - Mesopotamia, there is no bones about it, Mesopotamia, then Arabia. Arabia may well become as much of a quagmire as Britain howeverm unless they use client states/have some luck/integrate the Bedouin (yay, Bedouin light cavalry supporting the Romans, yes pls!). At a push you could see Axum/Ethiopia enter the Roman sphere of influence, and perhaps client state status - but that would be a very rosy situation indeed, unless we see the Empire survive intact.