Leaving aside the new paint scheme for a while, I do think EI needs to look carefully at its s/h fleet operation in the near future. Repainted or not, the A320s (particularly the 'CVs) aren't getting any younger and the Cityjet and Stobart fleets are a kind of mix and match of various different types - Embraer 190, 195, ATR42/72 and RJ85. Although I know they're operated by different airlines, it doesn't really give an impression of a streamlined operation or project and I think it is time that they started looking seriously at getting their subsidiaries to rationalise on a smaller number of types.
The new A321LRs ("32Qs", as EI wants us to call them) can handle the high revenue short haul routes, but is the 320 the right vehicle for the others; would it not be best to trade capacity for frequency, especially as they want to develop the "DubHub" even further. Maybe a 140-160 seater would be more suitable, then an 80-100 seater for regional routes and perhaps a smaller type for domestic routes and less busy UK regional routes.
Whatever solution is reached, the current set up really is a bit of a mash-up. Not EI's fault of course, but it does need to be addressed, so that a consistent product can be offered.
I think you're seeing a problem that isn't there to be honest. The Stobart and CityJet fleet which operate for Aer Lingus are not a "mix and match of various different types" at all, it's literally two types, the ATR and the Avro. The Embraer fleet at Stobart don't operate for Aer Lingus in any capacity, they're currently doing BA CityFlyer routes and some remaining Flybe business. The Avro is a special case, CityJet needed to offload the LCY route and Aer Lingus was the obvious choice, the Avro just happens to be the only LCY capable aircraft CityJet have.
Giving the "impression" of a streamlined operation is nowhere near as important as actually having one. The current Aer Lingus franchise operation is as streamlined as it gets and works perfectly for the mission it is tasked with. If and only if that mission is changed by Aer Lingus will fleet changes such as regional jets be considered and that could potentially happen when the combined Air Nostrum/CityJet group bid for the 2022 contract via their Hibernian Airlines startup.
They're also not subsidiaries so Aer Lingus can't dictate what fleet they operate, that comes into play during the bidding process and you certainly can't tell two competing airlines to streamline their fleets in order to create a desired image!
As for the A320 fleet, I agree it obviously needs to be updated sooner rather than later but I think the A320neo is the most likely and sensible option. It keeps the mainline operation simple with franchise operators picking up the thinner routes to feed the DUB HUB. Going for a 140-160 seater, then an 80-100 seater for regional routes and an even smaller type for domestic routes as you suggest would complicate the operation entirely and would even undersell Aer Lingus' core European business which is still very much A320 territory. The thinner routes and underserved markets that Aer Lingus struggles with in Europe could be dealt with by another franchise operator or an expansion of the current franchise deal, which brings us back to Hibernian and the 2022 renewal...
I think for the future mainline will stay Airbus A320 focused while franchise operations can be whatever is most suitable for the role they are tasked with, providing flexibility, frequency and feed.