What are ways to create "simplifed" Chinese characters?
Wikipedia's Simplified Chinese page has a section that outlines all the types already. Quoting it a bit:

There are four methods used:
Structural simplification of characters
Under this method, there are the following:

Replacing a character with another existing character that sounds the same or similar (e.g. 乾、幹 → 干)

Using printed forms of cursive shapes (e.g. 長 → 长)

Replacing a component of a character with a simple symbol (e.g. 對 → 对)

Omitting entire components (e.g. 習 → 习)

Further morphing a character after omitting some components (e.g. 歸 → 归)

Preserving the basic outline or shape of the original character (e.g. 飛 → 飞)

Replacing the phonetic component of phono-semantic compound characters (e.g. 鐘 → 钟)

Replacing some arbitrary part of a character with a phonetic component, turning it into a new phono-semantic compound character (e.g. 曆 → 历)

Replacing entire character with a newly coined phono-semantic compound character (e.g. 驚 → 惊)

Adopting obscure ancient forms or variants (e.g. 電 → 电)

Adopting ancient vulgar variants (e.g. 陽 → 阳)

Re-adopt abandoned phonetic-loan characters (e.g. 後 → 后)


Derivation based on simplified character components
This is where components that have been simplified are also simplified when they are used as radicals; e.g. 單 → 单 so therefore 彈 → 弹
Elimination of variants of the same character
E.g. 夠 → 够, 豬 → 猪. Note that the goal here is to reduce the total number of characters, which means that sometimes the simplified version has more strokes, e.g. 搾 → 榨
Adoption of new standardized character forms
E.g. 奧 → 奥, 虛 → 虚. These are structurally the same after simplification.

For more information see https://zh.wikisource.org/zh/%E7%AE%80%E5%8C%96%E5%AD%97%E6%80%BB%E8%A1%A8