I think the approach suggested by
@trainee4u is the right one. To add to his points, I have made a list bellow of crucial aspects to consider. Essentially, I think you should choose a new office location that seems to you to best fit the criteria you deem most important from those listed bellow; and then in the second part explain how reasons against it consist of the criteria the new location would not score so well in. This should be relatively unproblematic, as it is hard to think of any potential jurisdiction in which the different considerations would not pull in different directions. For instance, if clients in a given location are willing to pay high legal fees, the legal market is likely to be more saturated than other places and costs of setting up shop will also likely be higher.
Now, to list the factors I would consider most important:
- Projected growth: Which regions in the world are projected to see significant growth in their legal markets? Here, there's many that come to mind, such as Singapore, Brussels, Sao Paolo, Boston, and Texas. You can further research this topic to determine what regions are likely to see the most growth and which regions have the highest potential legal fees.
- Practice area and sector expertise: In which regions in the world would the firm's most well-known practices have the most demand? If your firm is transactional-focused Singapore might make sense, whereas if the the firm does a lot of competition/competition litigation work, Brussels might be the better choice.
- Legal market conditions: A relevant factor for consideration is also how difficult it will be for the firm to establish itself as a serious player in a new region. Establishing an elite office in New York might be substantially more difficult than doing so in Sao Paolo, as seen by the struggles the Magic Circle firms have faced there.
- Firm strategy and current network of offices: Does the firm prioritize revenue growth or retaining profitability? What is its current geographical reach, and what is its general international expansion strategy? A firm like Sullivan & Cromwell will normally focus on the highly-profitable legal markets in developed economies, while a firm like DLA Piper will have to ensure coverage of developing economies as well.
- Legal fees: as indicated above, another crucial consideration is the range of billing rates clients in that jurisdiction are willing to accept. In developing countries companies expect to pay significantly lower fees than in the US and Western Europe. Thus, setting up shop there can result in a dilutive effect on profit pools.
- Current client demand: for many firms who consider expanding a crucial point is whether current clients have any operations or need for legal services in that foreign jurisdiction. If you already have an established relationship in London or the US, it is a lot easier to win mandates from a given client in the new jurisdiction rather than winning mandates from new clients (where the firm would have to impress in a pitch to convince them to leave the firm they were previously working with). Furthermore, expanding in the main jurisdictions where your current clients need advice is a way of reducing the risk of them being won over by other firms (as a firm who advises a firm for a mandate in, say, Spain, might as well impress them and then persuade them to give them UK mandates as well). These two factors were central in Brad Karp's decision to invest as much as he did in Paul, Weiss' unprecedented expansion in the City- the firm was reportedly facing increasing demands to have a top London practice form Apollo, one of its most important PE clients.
- Cost of setting up shop: depending on the state of the legal market in the jurisdiction and the method of expansion chosen (acquisition/merger of a local firm, lateral hiring, organic growth or a combination of the three), the costs of investing in a new office can differ immensely. Establishing a top office in New York for instance is a herculean task, as the combination of sky-high associate and partner salaries and the size of competitors requires a huge amount of invested capital (which, among other things, explains why the Magic Circle firms have found it so difficult to compete with the US firms on their home turf).