You're still thinking about what to target, Danny
I'd suggest it's more about your own capabilities, knowledge, and interest. All three being important for different reasons.
But some thoughts.....
Poker - Don't even bother, you will never be able to compete with the big sites that have been around for 15-20 years and still rule the roost. There is no sign of Google going outside of the top names...and no need for them to do so either. All poker queries are answered (well) by those sites.
Bingo - We saw a couple of recent posts...big market. But boring as hell! Could you really write the same stuff about 100 bingo rooms or the difference between 85 and 90 ball bingo!!?
Sports - Depends on how you intend to tackle it, and how you could do that in a way that sets you apart/adds value. Tips, no. Seems to have been proven many times that tips sites will struggle (although I'd wager there are some run by experienced members where that is not the case). There would be plenty you could do though to build a desirable sports betting info site.
Which leaves casino. And, for me, that would be your best target
if you can build a site that can compete. There are many aspects you could focus on (begins with a B, for example), and new names do pop up in the SERPs still (which
indicates G will give some leeway to newer sites). Also, some of the bigger historical top sites seem to have dropped away
to an extent.
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For all of them, you are battling guys with deep pockets, guys with teams, guys that are able to work 16-18 hours a day solely on the site, etc
And you have to work out at the beginning - whatever target you choose - is there actually an exploitable market for what precisely I'm targeting.
So, don't underestimate the size of the task. But if you don't try, you definitely don't get, unless luck intervenes.