Masters of the Unseen Tower
DMG PG 283 has some general guidelines for spells.
Cantrip Single Target: 1d10 Multi: 1d6
1st level: Single Target: 2d10 Multi: 2d6
2nd level: Single Target: 3d10 Multi: 4d6
3rd level: Single Target: 5d10 Multi: 6d6
4th level: Single Target: 6d10 Multi: 7d6
5th level: Single Target: 8d10 Multi: 8d6
6th level: Single Target: 10d10 Multi: 11d6
7th level: Single Target: 11d10 Multi: 12d6
8th level: Single Target: 12d10 Multi: 13d6
9th level: Single Target: 15d10 Multi: 14d6
This table leaves a lot to be desired. Any number of spells available in the PHB fall outside these guidelines. So… If you have a spell idea, we will thrash it around and figure out what we feel is a balanced spell.
Once you have a spell concept approved, it takes your character 1d4*spell level weeks to complete the spell, as well as the cost in gp normally associated with scribing the spell. Halfway through that process you create an experimental version, which can be prepared and cast. If you cast the experimental version, you roll as would a Wild Magic Sorceror, and the spell may have unintended side effects. Once you reach the end of spell development period, no more side effects.