There are two main issues in this question.
(1) There are two actions that are being suggested and these should be parallel: (a) to decrease their debt (b) to increase their asset base.
(2) The use of the word should is redundant since the sentence (with the words require that) already carries the implication that banks need to take certain action.
- the word should is redundant since the verb to require and the context of the sentence indicate that the suggested actions should be taken
- the phrase it may be a good idea requiring that... is not correct as a verb (i.e., to require) is needed
- the phrase the decreasing of debt and an increase is not parallel
- the infinitive form of the verb to decrease cannot be used after requiring that all large and highly leveraged banks as it is awkward and not idiomatic
- the phrase banks ... to decrease ... [to] increase is parallel since both verbs are used in the infinitive form (the second to is implied); the unnecessary word should is omitted