no more 'submethod' - radio buttons will be name="payment_method" value="cc_visa" with fully specified string.

definePaymentMethods will define a single nested array:

$payment_methods['cc'] = array(
	'validation' => array ( 'foo' => true ),
	'countries' => array( '+' => 'ALL' ),
	'children' => array(
		'visa' => array(
			'validation' => ( array( 'bar' => true, 'foo' => false ) )
		),
		'mc' => array(
			'countries' => '-' => ( 'BR' )
		)
	)
)

Can override in localSettings with a gateway global
$wgGlobalCollectGatewayPaymentMethodOptions['cc'] = array(
	'countries' => ( '-' => 'GB' ),
	'children' => array(
		'visa' => array(
			'currencies' => array( '-' => 'JPY' )
		)
	),
	'selection_weight' => 5
)

new class PaymentMethodFilter will take metadata, gateway global, and set of selections (country, currency, maybe top-level method), and return a list of available payment method codes (with selection weight?).

FormChooser (renamed GatewayChooser) will call PaymentMethodFilter with each gateway in turn and
select from the gateways with non-empty lists, ranked by selection_weight then by number of different fully specified submethods

Somewhere in the form rendering pipeline we also call PaymentMethodFilter to get the list of submethods to show

Bonus: we might be able to get rid of Special:FundraiserLandingPage
when no top-level method is specified, paymentMethodFilter can return a list of top-level methods available in the user's country.