Would it have been a viable strategy for Great Britain to poach Spanish colonies? Maybe starting with the Philippines, Florida, or Argentina.
Yes. The problem Britain had post-7YW was exuberance that led them to be foolhardy about the European alliance structure. While dominant at sea, they failed to culture an alliance with any mainland power. That meant that France and Spain could concentrate on naval expansion without worrying about their armies too much and could successfully challenge Britain during the American Revolutionary War. While they had burnt bridges with Prussia, Britain could have developed an alliance with either Austria or, especially, Russia, but they were unwilling to make the concessions necessary to do so. If they had, France would have been locked down on the continent and they could have defeated the Bourbons at sea more easily. Florida was already gained in the Seven Years War, the EIC captured Manila during the 7YW (but failed to keep it due to giving an easy peace to France for domestic reasons), and the River Plate was a target, so all of these are viable captures.
I'm not sure if more land is really what Great Britain needed though. Maybe there's something parliament could do. Maybe a policy which encourages the development of existing territory would possible. Maybe more propaganda would have been helpful. After all, pamphlets played a decisive role in the English civil wars of the seventh centuries. What writers put the same amount of effort into promoting colonial expansion? Companies could even profit off of ads for colonization like occurred at a later date. How about ads from the British East India and British West India companies? Both to encourage the purchase of their products, increase labor, and garner political support for favorable policies.
The idea of state-led colonial expansion was largely anathema at this time. There is a limit to the administrative capacity of the British state and a big concern after the 7YW was whether they could manage all the land they had gained. As for the East and West India Companies, they DID lobby tremendously for both more funds and freedom from political interference, and were tremendously successful at it until the Great Reform Act in the early 1830s.
Furthermore, parliament might be able to create laws which encourage exports to wealthier European countries. Maybe some sort of "British Mainland Trading Company" would be helpful. Anything with caffeine or tobacco was likely to sell.
European trade was the majority of British exports until well into the 1800s. Forming a trade company monopoly would not only be politically impossible, as it would hit a lot of existing companies, it would also reduce trade due to the desire of monopolies to limit sales to keep prices up.
Alternatively, if there were some sort of political or religious conflict which shortly precedes otl economic conflict, then Britain might retain its colonies as it did with the civil wars of the 17th century. Then again, this might lead to even further division than in otl. Still worth considering.
Suppose there's a special division in parliament which serves the colonies for a designated period of time. That way, colonists are represented more directly. Nothing seems more alien than a distant shadowy government enacting laws without these leaders ever being seen in person. Seeing members of parliament in person would make the colonists feel more like they're a part of Great Britain. A visit from a prince or princess might help as well.[/QUOTE]
Representation in parliament (as for example, was planned by Pitt the Elder) would certainly have done a better job of improving dialogue between the American colonies and London. A major part of the breakdown was that both sides simply didn't understand the other's position very well, due to limited touch points (Franklin being one of the few). It would also have blunted the "no taxation without representation" argument, which was so successful. Of course, dialogue is only a mechanism for solving problems, so it would have to lead to good policy to be successful. It's also worth pointing out that colonial representation would have given more influence to the Caribbean planters than the mainland colonies, as they were much more important economically to the UK at the time.