The Improved Familiar feat is almost a microcosm of the evolution of d20 System options across its most notable RPGs.
In D&D 3.0, the feat didn’t exist under the Core Rules, premiering in the Tome and Blood supplement. D&D 3.5 saw it added to the Player’s Handbook (as well as the SRD), and while it had a few expansions here and there (such as in Dragon #331, which allowed a PC to take the feat and gain an additional ability in exchange for keeping their existing familiar), Pathfinder 1E made its expanded list of improved familiars Open Game Content (just like the rest of the system), making its options much easier to collate.
One thing that’s worth noting, however, is that the Improved Familiar feat doesn’t always require you to take a new familiar in place of your old one. It’s entirely possible for you to take a standard familiar with a template (originally that was only celestial or fiendish, but more possibilities were added over time). While that can be a different creature altogether, there’s no reason to think that this can’t be the same familiar you’ve had up until now, imbued with planar energies that “upgrade” it to a planar being.
Taking that idea further, why not allow for the Improved Familiar feat to augment your familiar in other ways as well? Likewise, why do those template require such strict alignments on behalf of the familiar’s master? It makes sense that you need to be good-aligned to have a familiar with the celestial template, but limiting it to Neutral Good feels too restrictive. Clearly, some tweaking is needed.
To that end, here are my suggestions for alternative options (in addition to simply selecting a more powerful creature) for what Improved Familiar can do:
Upon taking the Improved Familiar feat, if you are at least 3rd level or higher in the class that grants a familiar, you may apply one of the following templates to it (with your alignment requirements as noted):
Advanced, aerial, aqueous, celestial (any good), counterpoised (any neutral), cthonic, dark, entropic (any chaotic), fey-touched, fiendish (any evil), fiery, giant, primordial, resolute (any lawful).
Any abilities which are dependent on Hit Dice use either the familiar’s Hit Dice, or your levels in the class that grants you a familiar, whichever is higher.
Special: If you have a subtype corresponding to a particular template noted above (e.g. the Good subtype for the celestial template, the Aquatic or Water subtype for the aqueous template, etc.), have a familiar, and have at least 3 or more Hit Dice, you receive Improved Familiar as a bonus feat, but only to grant the corresponding template. If you have more than one subtype, you may pick which corresponding template to apply to your familiar; once made, this choice cannot be changed.
At the GM’s option, a template with a Challenge Rating adjustment of +1 or less other than the ones above may be granted to your familiar upon taking this feat.
This allows for a much greater degree of customization, along with a greater range of who can select the various aligned templates for their familiar. Likewise, the special notation allows for characters from alien realms to have a templated familiar without costing them a feat (since otherwise it would be odd to consider, for instance, an efreeti wizard might have to carefully protect a non-elemental familiar on the Plane of Fire until he could earn another feat slot). We’ve also left the possibility open for a different template to be applied if there’s one that’s not on this list but would otherwise be appropriate.
Hopefully this makes your familiar feel a little more new.