Well they've taken a step towards compatible spellcasters already with the current iteration of how spells are prepared and cast. Imagine there is a single spell progression chart for all classes. Now consider that the number of spells you can prepare is 1 + your class level. If you are an 18th level Wizard you prepare 19 spells and can cast according to row 18 of the chart. If you are an 18th level Cleric then you do the same. If you are 9th/9th split between the two, you only get 10 Wizard spells and 10 Cleric spells, but you cast according to the single unified chart. Caster level becomes additive, similar to attack bonus. If you want to mix in a 3E Sorcerer, they would need to use the same chart, but they might prepare only 1 + 1/2 level spells for Src levels, and in recompense get additional casting slots according to the unified chart for the Src levels. So an 18th level Src has twice as many spells as a standard caster, but half the variety. A 9th/9th Src/Wiz has 5 Src spells, 10 Wiz spells prepared, slots according to the 18th row of the chart PLUS slots according to the 9th row of the chart. (Note my Src is overpowered - can be adjusted to taste).
Intriguing concept. I see issues with the 9th/9th wizard cleric. How do you determine what class spells are on which level? Additionally, I see it somewhat limiting this way. If you say your 9th level spells come from the cleric side you wouldn't be able to say they are wizard spells the next day. They would always be cleric spells - otherwise a multiclasser would have incredible flexibility. I'm honestly interested in how you would handle those issues.