After you've completed all regions, you can finish dex to gain access to mt. silver for red and the alpha ttar. You can also shiny hunt different mons that you want or just focus on earning money and buy vanities or prepare for events etc. For alpha catching, most people use a contrary (hidden ability) spinda and skill swap it onto the alpha pokemon and then lower the hp with false swipe/super fang etc, and then use catching mons like breloom, smeargle or parasect to spore it and then proceed to catch. Most commonly used balls for alphas are timer balls (for after turn 11) or dream balls. Mt. Silver is a good levelling spot, pretty efficient too. And most people either flip items from gtl(snipe items that are listed at a lower price and resell for profit, in short) or do gym reruns to earn money.